If you really wanted to attach static meshes, I’d put the “door socket” ( maya locator) on the main body in Maya, and the door would need to have a pivot that works with the socket. Then in Unreal you would parent the door under the body, where the socket is.
FBX Static Mesh Pipeline | Unreal Engine Documentation
“… if I re-import the meshes everything I did so far will be overwritten and I will have to redo it all over again”
Consider working in a way where re importing a mesh does not break everything.
Is there a reason you want to go this route instead of rigging the car with joints?