He wants to use a door in an entity. i.e. like you would with a segment, but of course as default you cannot cut non-segments as you would a wall etc.
If your making your entities yourself, it's very easy to set animated doors into them by leaving a door sized hole in them when you create the entity, but as a rule normal doors will not cut into an entity.
You would (could?) get some horrid collision errors cutting into an entity as you would a segment I believe anyway.
I only use this for fun and have no interest in making commercial games ^_^