Possible but more often then not not practical. Your floor will be higher, there will be more weight on your flooring system and the existing tile must be in dam near perfect condition for the new tile. Even then it may not last long without a lot of cracking at the grout lines.

Hacks will do anything for money.

If you are talking only about the tile in the tub or shower surround (walls), you can glue a solid-surface shower wall like Corian or Swanstone over them sometimes (depending on the manufacturer's specific requirements). Then you get a water-tight tile wall system if the contractor seals the seams properly. You would use a solid shower receptor and remove the old shower floor or tub.

http://www.mapei.com/US-EN/Tile-&-Stone-Installation-Systems/Bond-Promoting-Primer/ECO-Prim-Grip