I was just playing around w/ that (literally, w/in the last hour ) If you look at VEHICLES.IDE at the last column for
each entry, you'll see mostly "0"s, a few "1"s or "2"s & the rest "-1"s. This sets what modshop you can take your car to.
Ranxorex caught that one. I've added a couple cars to Transfenders & they mostly seem to work.
WARNING: I can paint bikes, but the hydraulics don't work at all (game still runs, just no effect) & nitrous will crash the
game. Monster, MonsterA & MonsterB still come up as un-moddable.
RULE #1: BACKUP EVERY FILE YOU PLAN TO EDIT!!!
The short version is, no, I think you have to pick 1 shop for a car to go to. The only way to make that work is to find a way
to put all 3 in the same garage.