I've just read that the jjos2xl support velocity switching. You can use this to create a program that will play different samples of the same note (like I said soft/mid/hard) depending on how hard you play the pad.
To map the different strings on the pads exactly is not that easy (if you really want to emulate a bassplayer 100%) cause you'll need to know which string is played with the your specific bassline. Some "moves" from one note/string to the other are not really possible or a bassplayer wouldn't play it that way cause there is an easier way to play it. So you'll need to know which note is played on which string and map the specific samples to the pads.
But to be honest this method is an extreme example. The MPC is not really made for such a program. That doesn't mean it can't be done but its a lot of work and you might still be satisfied with a much easier and less complicated way of mapping even with a bass. To get a realistic paino multisample program would cause much more work.
I don't know jjos2xl so I don't know how to map the samples and if it's possible to map a sample to a range of pads/keys.
Calling Dr. Nym

peace