You're better off to buy a hardtop, it would be easier in the long run. No, you can't use the hardtop seals afterwards, the design is totally different. After all the fab work - and that's going to be huge, it would be easier to cut off the roof and weld on one from a hardtop - you'll still need new seals, new roof drip rails, b-pillar exterior panels, and headliner.
The fab work would be brutal... notice that the roof doesn't touch the body except where the bolts are. So you'd end up adding material all the way across the front and the back.
Lastly, what's the point? Why not just leave the roof bolted in? Why not leave yourself the option? I mean, if only get laid 5 times in 4 years, I'm not going to hack off my unit for weight savings.