It's definitely possible to add extra critters - though a vehicle may be a problem - it depends on what you want to do with it. I'm not sure if you'd be able to add a working vehicle that you could get in and out of, and also "drive round" on a local map. You could definitely do one or the other though.
So you could :
- add a vehicle like the current car, or other vehicles in 2238 (which appears on local map as static object, but is not "driven")
- add a critter which looked like a car, i.e. moved round local map, attacked, had loot - but it would technically be a "creature" or "person". It may be a problem to make a multi-hex critter.
You will need to edit bits in these folders/files (and probably more):
\Server\proto\critters
\Server\data\CritterTypes.cfg
\Server\dialogs (look at ones starting mob_xxx.fodlg)
\Server\scripts\_defines.fos
\Server\text\
I don't know enough to tell you the specifics on how to do more than that though. Hopefully someone else knows more on the subject and can tell you. If it's still working,
www.fonline.ru used to have a lot of answers on how to do things like this.