James,
Simple, just take the pen tool and draw the shape that you need around the footage. Animate the mask path if the footage moves.
For the doorway you could probably even use the rectangle tool, which will draw the sharp corners (you can change the degree of sharpness) and keep the correct 90 degree angles for you.
Grant Swanson
Visual Effects Supervisor
Video Apex - Minneapolis, MN
videoapex.net