Welcome to the Forum!
You should be able to do this, no prob, as long as you know the Bank Select MSB, Bank Select LSB and Program Change number of the user voice.
The voice lists are text files with a simple structure. Make a copy of the original voice list file just in case something goes wrong and you need to start over. Then, open the voice list (or its copy) in a text editor. No fancy tools.
Study the file structure. Compare it with the Data List PDF for the keyboard. Get a feel for the structural pattern of the data. Then make a single change and try it out.
I would suggest creating a new group of voices at the [g1] or [g2] level. Create something like:
[g1]User Voices
Then create new voice definitions using the Bank Select and Program Change number, e.g.,
[p2,2,0,113]Regular CP80 Stage
The "p2" tells the tool that this line is a voice definition. The "2" is the nesting level of the definition. If the definition is in a [g1] section, then use "p2". If the definition is in a [g2] section, then use "p3".
The next three numbers are Program Number, Bank Select MSB, and Bank Select LSB, e.g. 2, 0, 113 (above).
The stuff in square brackets is followed by the voice type (S.Art!, Cool!, Regular, whatever) and the voice name.
I'm sure the Yamaha tool just snarfs data from a voice list data file and creates menus from it. As far as voice numbers and voice name, it's just garbage in, garbage out.
Please give it a try. Report back. Be the first on your block!
All the best -- pj