I'm trying to add a box to a car's collision model (specifically, utilitruck3), but I can't figure out how to do that. The "chassis [COL]" node that seems to have the preexisting collision model doesn't appear to be a mesh that anything can be added to; it doesn't have anything to select at vertex or polygon level, and I can't attach another primitive to it. So what's the story?

I believe you would have to make the collision object seperate of the chassis. IDK how to do so. 

I tried making a new collision object misc_a [COL], but it just broke all the collisions. Anyone have other ideas?

An in-depth (although slightly techincal) tutorial on creating collisions can be found Here.

I'll try to help clear things up if you need after you read.