You want to convert your car? Bad idea.
Sure, take a sawz-all and cut off the existing top. Then you are on your own.
Buy a convertible. Converting your car, if its even possible will cost tens of thousands. The body is simply not engineered to go topless. The entire underside of the car would need to be reinforced so it won't flex and ultimately bend in half when you're driving it. Find other ways of enjoying your car the way it is. If you really want a convertible, and who wouldn't, sell what you've got and buy one. There's lots of nice older ones around if you shop a bit. Or do like I've done and have one of each... a GTP for the winter and a convertible ('94 Cutlass) for the summer. Life will be good.
Looking for a Used Grand Prix in your area?
CarGurus has 2,725 nationwide Grand Prix listings starting at $1,200.
Search Pontiac Grand Prix Questions
Pontiac Grand Prix Experts
Related Models For Sale