Nothing is impossible, and it doesn't requires any special software, but you must have access to a Psr-s770, to be able to record the wav samples from the Expansion.
It could be very time consuming depending on how many voices we are talking about, and how complex they are.
Also remember Psr-s770 have 158 mb memory and Psr-s950 64 mb.
If you have access to both of the keyboards,
it's gonna make it a lot easier to recreate the Expansion for the Psr-s950.
Note: You need a DAW (Digital Audio Workstation) to record and edit your wav samples, and
Expansion Voice Editor for recreating the Expansion.
How to proceed:1. Load the Expansion into Psr-s770.
2. Select a sound, and start to record from key of C6 directly into the built-in
USB Audio Player 2 sec long wav samples in key
C, A, F#, and D# for each octave, when you have recorded all 4 keys press the
Stop button and save the wavsample.
3. Continue the process with the rest of the octaves, and save the wavsamples in folders named (C6, C5, C4, C3, C2, C1)
Note: Depending on the sound sometimes it's enough with 2 wav samples for each octave (Normally C and F#).
When you have gathered your wavsamples, move on to the next step.
4. Transfer your wavsamples to the desktop on your computer.
5. Open your DAW, and drag and drop the wavsample into your DAWs recorder.
6. Now follow my video
https://www.youtube.com/watch?v=KAk3oSrTv54&t=50s on how to cut, edit and loop each sample you have recorded, and how to assign the wavsamples into Expansion Voice Editor.
As I said nothing is impossible, and it doesn't requires any special software to achieve this.
Regards, Jan.