I'm not sure what the problem is here. It is definitely possible to make the body of the car as root of your scene. Any node you want can be the root.
When creating a new scene, you have choice between
Control, but you can also choose anything else with the button below.
And when you already have a scene with nodes inside it, you can right-click on one of them in the scene tree dock, and in the context menu choose "Make Scene Root".