You can load samples into the YEM software on a PC, create voices from the samples, export the voices as a pack, then install the pack to the keyboard.
So, this means the answer is yes - you can. But it's a little fiddly since you can't do it on the keyboard itself, and the results will vary in quality. If you were doing it all the time it would get frustrating, if it's now and then you can probably live with a little trial and error each time.
There's also a limit on the total size of the loaded waves which varies by model. For S975 it is 768MB. This cannot be increased.