You'll have to sort of remake the level as a new map to do that. The files are already included for the shipped levels. You'll just need to copy the world files into a new level, add a vehicle spawn in ZeroEditor for whatever vehicle you're going to use, and edit the lua in two places. That's the short explanation, anyway. I haven't done any of this myself, yet.
"Do you not know the pain you will suffer for this impudence?" - Ba'al