Is this sort of what you are talking about doing on the Roland BK-9? Here is an example of assigning audio clips that are edited with Cubase and then assigned to the multi-pads on the PSR-S950. These audio clips are exported into the PSR-S950 instead of recorded directly from the keyboard itself. There seems to be quite a bit of syncing capabilities going on here with the Yamaha platform.

http://www.youtube.com/watch?v=5KeLJenWFbs