George - Try this - First select a user voice to edit (pick one you dont mind overwriting or make sure you back up the user voices first). Then press Exit for the main menu followed by F7 (Edit voice). If you then press the various voice family buttons you will then see all the GM voices listed and you can then select one of these. You then press EXIT again and find that you are editing the GM voice rather than the user voice you selected. This can then be saved into any User voice slot. I've written this from memory whilst at lunch at work but I'm pretty sure that's how I did it.
I did recover a piano voice this way for the user voice that I'd overwritten. There are several other "features" that are not documented in the manual either which leaves plenty of scope for another video or user guide by Dan0
Regard - Keith