I am struggling with your post to understand if you actually have a vector mask(with bezier handles and vector points), or if it may be a bitmapped mask(black and white pixels and possibly some gray.
If you have a vector mask, copy this, then go to the other layer mask, and paste it.
If you have a bitmapped mask, command click to make a selection of marching ants,then go to the layer you want to add the building, and fill the marching ants with white.