This example for Lenny's Mod Loader adds 50 (!) new police slots to the game, all based on POLICE2. The slots range from POLICE100-POLICE149. Please note that only POLICE100 and POLICE133 will have models (both the POLICE2 model) as I did not want to include models for all vehicles and two cars should be enough to understand how it works. Those two are the only ones that will spawn in-game by default. The data files are all set up so all you need to do is copy the model of your choice into the streaming folder, give them a name from POLICE100-POLICE149 and the next time you start the game, your new police car will be in that slot. Of course you can modify the entries for every slot, e.g. to change a vehicle's handling, but it is not necessary to get started.
Why 50 slots? Because why not.
I recommend to install this gameconfig first so the game can handle all the new cars.