It is not just a matter of importing a model into a game. You need to convert the model to the correct format. then the model has to have separate submodels (like the wheels) and even the separate body panels (for the damage simulation ingame, if it has such a feature). then you also need to write the config files or scripts that define the behaviour of the car, e.g. how it reacts to collisions, the handling characteristics, acceleration and braking, engine power, drag, and a whole lot of other stuff. Since you don’t seem to have the background in modding, i can not recommend trying to do that as this will cost you a lot of time. a lot. you are better off searching for mods on the internet.