You need to create a new EXS24 instrument for each patch/preset.
So, if you want one stab sound to be assigned to one instrument, then you need to create different instruments per stab....
or if you are willing to experiment with your workflow....
ok, for example, you drag a bunch of different stab samples into one exs24 instrument editor. You assign them all to individual notes.
When you want to preview a different sound for that "stab" part open the piano roll and select all the events on that note.
(Easiest way is to click on the corresonding piano key on the left).
Now you can move all these events up or down by pitch using the key commands "Event transpose +1 / -1"
Mine are set to Option-Up arrow and Option- down arrow respectively.
This will actually save you the time of creating lots and lots of individual exs24 instruments and save the time of loading them up while previewing...
Let me know if that works for you...