I believe you can only change existing vehicles, not add new ones, so your choices are police, police2, noose, polpatriot (if you want a truck or SUV, cars don't handle as well in this position) and fbi. You also have nstockade and stockade [might be polstorckade or pstockade - can't remember off the top of my head] however those are going to be slow and roll over around every corner.
What I have done is made a copy of my vehicles.img file and I just change the name of it depending on what is in it, so if I play with a certain group of people I have certain cars that they roll with. For example I would just have to rename whichever set I want to use to vehicles.img and change the current vehicles.img to whatever it contains, say I have vehicles-orig.img which is the vanilla GTA 4 vehicles, then I have vehicles-lcpdfr.img for use with people from this site, etc, etc.
Hope that helps you a bit.