[Psycho]

U can make a vehicle thief by adding the following lines to an infantry type :

Infiltrate=yes
Thief=yes
VehicleThief=yes

i didn't tried but think u can make a seal as hijacker so that he can steal both land and naval vehicle types

I can understand what u say about map triggers. I didn't do any map ever but i read some tutorials those r written for novices. May b u have to put a celltag inside the building u attempt to capture with engineer. Now make a trigger that will b fired when the engineer will pass/enter the celltag. That trigger can b made for a reinforcement or to change a building's ownnership.