Hi Chas,
Welcome to the PSR Tutorial Forum!
Unfortunately, the PSR-S900 does not offer the option of installing expansion voices. This is only possible from S950 (only one pack at a time in the now discontinued .yep format). The models from S970/975/770/775, PSR-SX models as well as Tyros5 and Genos) are compatible with the current Expansion Manager (YEM) technology.
On the PSR-S900 you can only change the internal voices in the Voice Set and/or in the Mixer and save as so-called User voices (or in registrations). On the following page you will find a lot of User voices (e.g. .vce) that you could try out. You can access them directly from the USB stick:
>>>
https://psrtutorial.com/sty/OF/voice.htmlPlease note that such User voices can only be used in keyboard parts (Right, Left), but not in styles, multipads or MIDI files.
Important: User voices never contain new samples, but are always based on the samples of the original Preset voice (therefore only contain the changed Voice Set parameters).
Hope this helps!
Best regards,
Chris
Edit 2022-05-02:
Link fixed