Z-Buffer Images using bit-blt or DirectDraw

If you ever examined an object in "The Sims" it has a Color Bitmap for the object, an Alpha Channel for transparency and a Z-Buffer Image so that a sim can sit on a sofa without it looking like the edge of teh sofa under the sim sitting on the surface. How could I use these 3 kinds of images to create something like a game where you place thiings down on a map and it will have a 3d look to it even if its in 2D, in bit-blt. If you could help me do this in DirectDraw it would be appreciated.