I think I understand what you're saying - but it's difficult for anyone without Motif experience to know what a "voice" is - since Montage has no "voices". It's better to refer to Montage's "stuff" using terminology that matches Montage -- since you'll find these terms either printed on the keyboard enclosure, within menus of Montage's graphical user interface, and/or inside Montage's documentation. "Voice" fits for Motif - but once you load a Motif file into Montage - the end result ceases to be a voice and becomes a PART within Montage. Of which there are 16 possible PARTs - 8 of which you can "play'" with the piano keys at the same time.

Also "record" can mean lots of things. I'm not sure if you have a tape recorder you're using to record, a DAW recording audio, a DAW recording MIDI, the on-board Performance recorder (MIDI or audio), or ... ?

Performance Recorder as in https://www.youtube.com/watch?v=ku39jiK7x8Y

Regardless what you're using to record - if you want to add other PARTs to combine Motif XF converted performances (which should "mostly" end up as multi-PART performances on Montage) or Motif XF converted voices (which should "mostly" end up as single-PART performances on Montage) - then you need to edit the resulting Performance from a Motif XF conversion. I'm assuming you know how to find the Motif XF content (recall a Performance that was sourced from Motif XF data). Once you've loaded the performance - press [PERFORMANCE] (HOME) to get back to the home screen.

This is roughly what the home screen looks like:

You would add different PARTs to the existing Motif XF content by clicking on the [+] (plus) sign and using CATEGORY SEARCH to find a PART you want to add.

For more details - this tutorial is helpful:

https://youtu.be/WIumqa4bEZA?t=44

I have started the tutorial where an example PART is added to an existing Performance which was previously recalled.