Can you tile over existing bathtub tiles. If so, how ?

Well yes and no. You could but you'd need to break the surface with a grinder wheel or a belt sander with aluminum oxide paper. You need to creat a surface for your thinset to bite. I would not recommend it if you have spigots or shower valves coming through the tiled surface. They usually will not allow for a thicker wall. Maybe tell us more details and what you are planning. Also, what kind of tile is there now, ceramic, porcelein, plastic, stone...