There's a few ways to interpret your question.

"It is possible to load a single sound in a library in to the montage?"

I'm going to interpret "single sound" as meaning a single Performance.

The way a Library is created on-board (without using computer software) is that you take everything inside the User bank and create a Library file. Then load that Library file into a Library slot. If your User bank has only one Performance - then you can save a Library file and then load that into a Library slot.

... if what you mean is "can I load a single sound into a library and then later load another single sound and stick it in the same library?" And if that's what you mean, then no.

The Library bank is like fired pottery. Once you establish a library slot - it's a done deal. The only way to get anything different into that same library slot, you have to smash the pottery (delete the library) and then fire up a new library.

The User bank is like fresh clay. You can tear off a piece or take a new piece and lob it on. That's the assembly area for your clay before you kiln fire the user clay to form your hardened library (if you wish).

It is possible to load a single sound in a library in to the montage?

Background
A MONTAGE “Library” with a capital ‘L’, is a MONTAGE term that describes data that you have installed to your instruments 1.8GBs of FLASH memory. Each Library can include a maximum 640 User Performances, 256 User Arpeggios, 32 User Curves, 256 User Motion Sequences, 8 Banks of 256 User Live Sets, 8 MicroTunings and 2048 User Waveforms.

You can review your current USER Bank data by navigating to [UTILITY] > “Contents” > “Data Utility” > you have Folders for Arp, Live Set, Motion Sequences, Waveforms.

You can create and install 8 Libraries in your Flash memory. Once a Library is installed it remains in your instrument’s memory until you directly DELETE the data, or you initialize your entire instrument. In other words, you cannot overwrite a Library by simply loading data, you must purposefully Delete or Initialize all memory to remove them. They become as permanent as the Preset Performances, Arpeggios, Curves, Motion Sequences, Live Sets and Waveforms (which are permanent-permanent, in that they can never be lost).

You, the user, can assemble User data that can be “saved as...” a Library File — if you want to install it as a semi-permanent data to your instrument. You would do so with data you have created or you have purchased.

A MONTAGE Library File .X7L or a MODX Library File .X8L can be installed to one of the eight Library locations.
From the data you assemble in the USER area you have the option to create a USER File .X7U or a LIBRARY File .X7L by going to [UTILITY] > “Contents” > “Store/Save” > you can set the “Content Type” to USER FILE or LIBRARY FILE.

A User File, when created, can restore your User Bank data to the instrument’s RAM (Random Access Memory or work area). One User Bank can exist in RAM at a time. That means, loading a “User File” will overwrite the current User Bank.
A Library File, when created, can store that same data in a File Type that when selected to be “loaded” will *install* the dat to Flash memory. The process of installing to ROM (Read Only Memory) is often referred to as “burning” the data to ROM.

This burning of data into Flash memory is more intense than just regular loading and requires more time than loading the data to RAM. It is more like publishing a book... to continue the Library analogy. A published book is placed in the public Library... where it can always be referenced anytime you need to refer to it. Referencing it could mean playing it as is, using it to build yet other Performances, you can reference it in a Live Set (set list).
It is a ‘published’ work that includes many chapters (Performances, Arps, Motion Sequences, etc). And like the Preset data, is used as reference material.

If a published work is in your Library, you cannot easily make a permanent edit to the data. This does not mean you cannot edit it, of course, you can. But in the same way you can edit a Preset — your edited version of the data must be written to a USER Bank location. Your edits to a Preset Performance, does not alter the Preset permanently (it remains unchanged) your edit must be written (Stored) to a User location.

Same goes for data you have burned into a Library. You can edit it but your new version is stored to a User location. The Library data remains as it was.

In order to make a change to the MONTAGE Preset data requires a firmware update from Hamamatsu, Japan.
In order to make a change to a MONTAGE installed Library means you must use “Library Import” to move the data to RAM (work area), make your changes, and literally, make a new Library to replace the earlier edition.

In order to make a change to a published reference book in your public library, you need to remove the current version and publish an entirely new edition) a second edition to replace the first.

Saving Performances, one-by-one
Now all that said, a Library with one Performance in it is entirely wasteful (there is room for 640 Performance).

Since you did not capitalize the word “library”, perhaps you only want to save/load a single Performance. And that is possible by using either the computer utility called MONTAGE CONNECT, or the iOS app “Soundmondo”. These programs allow you to manage User Performances that are in your MONTAGE one-by-one.

MONTAGE CONNECT creates a .X7B File which is a MIDI Bulk Dump of the current Performance.
Soundmondo allows you to Capture a bulk from your instrument and stores it on your iPhone/iPad.
These will memorize the current Performance only... by that I mean, it does not include the actual data for the Arpeggio, it simply remembers which Arp location you used; it does not include the actual Motion Sequence data, it simply remembers which Motion Sequence location you used; it does not include the actual Waveform data, it simply remember which Waveform locations your Performance points to, and so on. It points to your permanent and semi-permanent ROM data.

It is highly recommended that you create your own a Libraries only after you have assembled enough data to make it worthwhile. While building up data... Make a bulk MONTAGE CONNECT or Soundmondo Capture document for each of your individual Performances. You can group them using the utility so that you know what User File they go with.

By making these individual captures of your own programs, it is much easier to assemble them when you’re ready to create a Library.

Alternative Method to Load a single Performance
If you purchased a MONTAGE Sound Library, and only wish to load one Performance from that LIBRARY FILE (X7L) here’s how:
You must install the entire Library, temporarily, to one of the eight Library locations.
Play through all of the Performances — make note of those you wish to keep.
You can then use “Library Import” to select the Performance(s) you desire (this “Library Import” function will look into the file’s catalog and will determine which data must be imported to make the selection play properly). This means if the selected sound uses custom Waveforms and Samples and/or if the Performance uses a specific user arpeggio all the data that is necessary to make it play properly is imported.

You place a check mark next to each Performance you wish to import to the User Bank, tap “Import to User Bank”... check all of the ones you wish to grab — this way MONTAGE can automatically remove any duplicate data.... your selections will be moved to the lowest numbered empty location in your MONTAGE User Bank. Now that you have extracted all the data that makes your Performance play, you can Delete the temporary Library.

Hope that helps. Thanks for the question.

Thank you very much for the reply! sometimes my memory is full and I just wanted to load a single sound (performance) from a library into my Montage.

If that's your intent, you may need to shuffle.

0) Generally - backup your entire instrument first because if you mess up - you want to be able to get back to "normal"
1) Delete a library to clear some space.
2) Install the new library where you want to keep only one Performance
3) Library Import the one Performance you want to keep into your user area
4) Delete the new library you just installed
5) Re-Install the original Library you deleted in step 1 just in order to clear space.

... it's possible that even the one Performance is "big" and doesn't fit with the step 1 Library. In which case, you'll need to do more optimizing to get that single Performance in.

At the end of the above steps - your new Performance that started in a Library ends up in the User bank. If you want to really have just that single Performance in a Library slot - then:

0) I would still backup the instrument first even though you're saving the user area
1) Save your user area to an X7U so you can restore it (and only the user area alone) later
2) Initialize only your User bank (not the entire instrument)
3) Delete a library to clear some space.
4) Install the new library where you want to keep only one Performance
5) Library Import the one Performance you want to keep into your user area - now your user area will just have one Performance
6) Delete the new library you just installed
7) Save a library file (X7L) - this will create a library with only the one Performance you wanted to place in the Library slot. It saves the user area to a library file.
8) Install the library you just created (the 1-Performance X7L)
9) Restore the X7U user file you saved in step 1 to return your User bank.

When you reach a point where the memory is full, no more data can be added. You must Delete some data in order to load any thing additional.
This is why it is very important to make sure all the data you install to Flash memory is data you use. It is entirely wasteful to keep Waveforms in memory if you never use them.

You should keep you .X7L files in a safe place. This way when you Delete one of them (to make room) you can restore it later. All your currently installed Libraries were placed there by a specific .X7L file — keep the individual installed File in a folder on your computer or on a specific USB stick.

I really appreciate your answers, thank you very much Bad Mister and Jason.