I've had to think about it... it's been awhile. I never owned a S950 or S750. There was two pieces of software... never used either one though.
The one pj mentioned is called YEP Voice Manager, which allowed you to create your own Voices. It was a hacked version and probably not to Yamaha's liking.
The other one was call YEP Style Organizer which is Public Domain and let you add/delete your own styles to a pack. Not sure if it had bugs in it or not. I have no idea where you might find them.
Joe H