You'll need a double-throw switch, i.e. one that can switch one output to either of two inputs. I'd get one with an off position as well (often labelled on-off-on in shops).
Before wiring it up, you'll need to check your car's wiring diagram to see how the existing interior lights are wired - you'll need to do yours the same way. It will depend if the switch is on the feed side or the ground side.
If the switch is on the feed side, connect as follows :
12v - fuse - switch 1
Door switch - switch 2
Switch common - lights - ground
If it's the other way round, connect it like this:
12v - fuse - lights - switch common
Switch 1 - ground
Switch 2 - door switch