Not that you're asking about FM-X exclusively, but ...

DX7 had this. Although FM-X sports some degree of compatibility with the DX7 patches, there are some exceptions. This is one of them.

There's not a way to do what you're asking for that I'm aware of without bringing in some external gear or software that will handle this for you (recognize legato playing and send the necessary between MIDI notes at some rate).

Alright, thanks. A strange omission given that it has existed in the past. Seems like it could be really useful for certain acoustic instrument articulations (but also synth stuff). Maybe there's some technical reason for it, but given all the ARP stuff that exists (which basically does something like this but in a different way) I don't really see what that would be.

Stepped glissando (as performed on a piano) is possible on your MODX — there are plenty of videos on YouTube if you search “glissando”. Thumb technique, two finger technique, etc., etc., different techniques for up and for down, etc., no big deal on a keyboard. Any instrument tuned to Equal Temperament will generate “stepped glissando” using the standard piano-technique.

There are both Waveforms (Strings, CelticHarp...) that are glissando and arpeggios (Darbuka and Sitar)
It’s funny, glissando is differently defined for different instruments. While a similarity can be drawn between a Harp and a Piano gliss, it is different from the Sitar and string glissando... and a percussion instrument gliss... Most percussion instruments do portamento a term which is often used interchangeably with glissando... but winds up being different on different instruments. A hammered or struck instrument typically does not do Portamento in a way similar to say, a bowed string instrument.

You can create your own Arpeggio - but remember an Arpeggio is limited to 16 unique notes.
Without knowing your application (how you intend on using it), an Arp may not be the best solution for your requirement. Since nothing could be easier than a stepped glissando, you may not need to automate it at all.

Create Your Own from a .WAV
You could record yourself (as Audio) performing the glissando using the exact sound and notes you desire to a USB stick, then use that data to create a MODX Waveform. it’s easy enough...
Call up the Performance
Press [RECORD]
Select “Audio”
Set the Trigger to a value of 1 — this will allow recording to begin when you begin the glissando. This is important, you want your waveform to begin playback when triggered with a Note-On.

Try to approximate the tempo and attitude you need for your musical use. Consider how long it should last and make your best guess about how it will need to sound to fit what you have in mind.
Once you have your recording, press STOP as soon as you’re done — because you have no ability to edit the Wav in the MODX, you want to create a recording as close to ‘useable‘ as is possible. However, if you need to edit it - load it into your computer and use a program (Cubase, WaveLab, etc.)

This 24-bit, 44.1kHz stereo .wav can be converted to a MODX Waveform, and placed as an AWM2 Element in a Normal Part or a Drum Kit Part.
From the USB stick you Load this .wav directly as a NEW WAVEFORM — this is done on the “Osc/Tune” screen of an AWM2 Normal Part’s Element or of a DrumKey within an AWM2 Kit Part.
You can target a specific Key to be the Key that sounds the original pitch of the recording.

Once your Waveform is assigned to a Part, you can figure out how you want to trigger it. It will be set so that when you play the Key with the original pitch assigned, it will play back your stepped glissando

I'm not aware there's a way for MODX to respond to two arbitrary legato piano key presses, either up or down, in a way that will "fill in" all chromatic notes between.

An application would be to more easily emulate guitar glissando where a player strikes a note then runs the finger up or down to a note "far away" on the fingerboard (same string). Pat Metheny's playing style is "slippery" and runs up and down like this "slurring" many notes using this technique. Playing the white or black keys is how one would best emulate this on a keyboard - but glissando would be a closer match as chromatic is what you're after and white or black keys is diatonic/pentatonic.

The start and end notes of the glissando would not be known prior to performance due to the improvisational nature.

Stepped glissando (as performed on a piano) is possible on your MODX — there are plenty of videos on YouTube if you search “glissando”. Thumb technique, two finger technique, etc., etc., different techniques for up and for down, etc., no big deal on a keyboard. Any instrument tuned to Equal Temperament will generate “stepped glissando” using the standard piano-technique.

There are both Waveforms (Strings, CelticHarp...) that are glissando and arpeggios (Darbuka and Sitar)
It’s funny, glissando is differently defined for different instruments. While a similarity can be drawn between a Harp and a Piano gliss, it is different from the Sitar and string glissando... and a percussion instrument gliss... Most percussion instruments do portamento a term which is often used interchangeably with glissando... but winds up being different on different instruments. A hammered or struck instrument typically does not do Portamento in a way similar to say, a bowed string instrument.

You can create your own Arpeggio - but remember an Arpeggio is limited to 16 unique notes.
Without knowing your application (how you intend on using it), an Arp may not be the best solution for your requirement. Since nothing could be easier than a stepped glissando, you may not need to automate it at all.

Create Your Own from a .WAV
You could record yourself (as Audio) performing the glissando using the exact sound and notes you desire to a USB stick, then use that data to create a MODX Waveform. it’s easy enough...
Call up the Performance
Press [RECORD]
Select “Audio”
Set the Trigger to a value of 1 — this will allow recording to begin when you begin the glissando. This is important, you want your waveform to begin playback when triggered with a Note-On.

Try to approximate the tempo and attitude you need for your musical use. Consider how long it should last and make your best guess about how it will need to sound to fit what you have in mind.
Once you have your recording, press STOP as soon as you’re done — because you have no ability to edit the Wav in the MODX, you want to create a recording as close to ‘useable‘ as is possible. However, if you need to edit it - load it into your computer and use a program (Cubase, WaveLab, etc.)

This 24-bit, 44.1kHz stereo .wav can be converted to a MODX Waveform, and placed as an AWM2 Element in a Normal Part or a Drum Kit Part.
From the USB stick you Load this .wav directly as a NEW WAVEFORM — this is done on the “Osc/Tune” screen of an AWM2 Normal Part’s Element or of a DrumKey within an AWM2 Kit Part.
You can target a specific Key to be the Key that sounds the original pitch of the recording.

Once your Waveform is assigned to a Part, you can figure out how you want to trigger it. It will be set so that when you play the Key with the original pitch assigned, it will play back your stepped glissando

Thanks for those suggestions. I would mainly want to use this to simulate a guitarist running his finger up the string before landing at the desired note. I think this would be rather difficult to do by actually playing it, especially on a weighted keyboard (I have the MODX8). I would also want these notes to be played legato style, because there shouldn't be an attack for each one. So I don't really think performing it myself is realistic, at the speed it should happen at.

Sampling this from an actual guitar would be an option of course... But then I would really have to do it for every note for it to sound good. I would want to be able to do this glissando up to any note. Just pitching one such sample across the keyboard would definitely not sound good. But probably this is the most realistic option. Will just take a lot of work! I would optimally also have separate samples for going down instead of up. And then to trigger the glissando I guess I would program the elements to play when either of the assignable switches are held.

I'm not aware there's a way for MODX to respond to two arbitrary legato piano key presses, either up or down, in a way that will "fill in" all chromatic notes between.

An application would be to more easily emulate guitar glissando where a player strikes a note then runs the finger up or down to a note "far away" on the fingerboard (same string). Pat Metheny's playing style is "slippery" and runs up and down like this "slurring" many notes using this technique. Playing the white or black keys is how one would best emulate this on a keyboard - but glissando would be a closer match as chromatic is what you're after and white or black keys is diatonic/pentatonic.

The start and end notes of the glissando would not be known prior to performance due to the improvisational nature.

Yeah, that's exactly what I would be trying to emulate here.