Try giving the units waypoints inside the building. If you put a waypoint on top of the building, then the waypoint should give you an option to choose which location the unit should go to. Of course, you'll have to experiment to find out which position is the one you want, since the positions are labelled by number instead of a description.
Using a waypoint will cause the unit to move there. The only drawback of this method is that if you need the unit to start there at the instant the mission begins, he won't be there yet. But, if it doesn't matter, then give this solution a try.