Well, I've come up with a way to have your own class.
This is handy when you need a place to put all your modded cars.
Simply open up Classes.ini and add a class at the end of the file.
You must'nt follow the flow, you cannot add "IDS_CLASS_16" if the last one is "IDS_CLASS_15", this is because you'd have to place the name of the Class in the english.ini file. Which BTW would be a PIA the edit since it uses a string list.
So name it whatever you want, like "Paul's Garage".
Then open up Cars.ini and replace whatever car's class you want to your new class!