Nothing is impossible.
I've retrofitted paddle shifters and it's a simple loom inside connecting the buttons from left to right and then into the slip ring.
In essence all the switches do is ground one wire for an upshift and one for a downshift
Can't see any reason why you could not chop and rebuild the harness to create the different selection you desire, but never tried it as I like the way it is now (push either for down, pull either for up)