Can you put a metal roof on top of an asphalt roof?

Yes you can. You will want to cut back any shingles that over hang the edges. Then put 1x4 slats on 19.2" centers parallel to the ridge of the roof make sure to hit the rafters when you nail down the slats. Do not use treated slats as they will react to the metal. Then screw down the metal the the slats.