Hi anotherscott,
With Genos and Tyros5 you can create Custom Voices in the YEM based on Preset Voices, if necessary also combined with new WAV samples.
With YEM-compatible PSR models, such as SX900/700, you can only create Custom Voices based on new WAV samples in the YEM, but not use samples from Preset Voices.
Note: CUSTOM Voices created in YEM can also be used in Styles, MIDI files and Multi Pads. USER Voices, i.e. Preset Voices edited directly on the keyboard, can only be used in Keyboard parts (right/left). (When using USER Voices for revoicing Styles, MIDI files or Multi Pads, the changed Voice Set would be ignored, i.e. only the underlying Preset Voice would be loaded).
Hope this helps!
Best regards,
Chris