Not sure this is what you mean so in case its not you can ignore it :D

But you could use the hieraki editor to link all parts to the main part for instant. Make the changes you need and then save the car to your library. Then you should be able to simply add the car again from the library and it will have the same features as the other one.

Another way is when you have imported your car and you have all the different parts etc. Make all the changes you need and then export it again as an OBJ including all the parts and then import that one again. In that case the model will be 1 obj with the changes you made before you exported it. You might have to set up materials again though, but im pretty sure it will maintain them, but anyway just beware that you might have to fix them again.