Ok first off the link you posted doesn't work for me so I'm basically guessing about your question.
If you are talking about the car door it'self being seperate from the chassis then there is no reason to combine it and it would be perfectly ok to have that as a seperate object.
If you want a way to create a door with same geometery as the chassis ,one option would be to select wireframe on shaded under the view menu in the view you are working in and then select the edges down one side of the hole in the chassis where the door would go and then extrude them out several times along the x axis matching the bottom verts as you go.then tweek the shape to match.
Another would be to select the edges both side and go to the bridge tool options and set the number of divisions to match and then apply the bridge this will give you minimum tweeking to do , once you have it bridged just right click and select the faces and under Mesh click on the extract option box and turn of seperate faces then estract those faces giving you a door you can then give thickness and shape too.