I use the piano board, and I'll try to answer what I can: it is possible to layer sounds using a combination of the plug-in voices and the 9000 voices -- ie, R1 can be the 9kpro's piano, and R2 can be one from the pluginboard. this offers nice fat electric piano possibilities -- a current fave is 9kpro's "Cool Piano" and the plugboard's "Delayed Dyno." trick is, only one voice from the plugboard can be engaged at a time. (Yamaha "suggests" in the manual that if more polyphony is desired, you can put an identical plugboard into the second slot)
the plug in voices can be saved in the registrations; they come right up as though they were native.
hope that helps.