Is it possible to place a fiberglass shower over tile without taking tile off?
Check out bath fitter. They put a whole fiberglass shower and tub over the old tile one in a day.
I would not recommend this, but it's definitely better than trying to place tile over fiberglass. There is a product that is pvc tile called DumaWall that specifically states that it is suitable to place over existing tile. It's an easy to install click lock product made for shower walls.
You might be able to find a kit and adhere to the tile with glue. Although I think this is going to be a pain in the neck to do.