I need some help putting some police cars into the game. Currently, its the default police pack. What should I do if I only want to add vehicles and not customize them? Thank you!

I don't understand what you mean. Have the vehicles without textures?

 

I don't want to customize them like change the features, I just want to add them to the game.

As in you don't want to replace vehicles like police, police2 and so on?

If so, then it's impossible to add it to the game. If you want to add a police vehicle, you will need to change one of the police models in Open/SparkIV. 

One thing you could do is pick a vehicle that you don't like and replace it with the vehicle that you want.

Please note, if you're going to do that, you will need to rename the vehicle files to that vehicle.

I also cannot ensure that ELS will work.

BACKUP THE ORIGINAL VEHICLE IF YOU'RE GOING TO DO THAT!