I have been wondering the same thing. I believe this is possible. One option would be to tap into the internal clock pulse, and see if you can run it from an external clock. Triggers may also be possible. Midi should theoretically be possible with a UMR2 midi kit, since the PSS-50 appears to have a matrix-key interface going on. But it would require figuring out how to make that work on your own pretty much, there is zero support for the UMR2 these days.
Anyway, I came across a PSS-50 recently and I am going to explore the potential for a clock in mod, that I mentioned. I’ll keep you posted if I have any luck.