I have not heard of anyone modifying a plastic window top to accommodate a glass window. Sounds like more effort than would be worth it in the end, and I doubt any reputable top shop would want to undertake that.
I have seen people quote anywhere from $250 to $600 to have a new plastic window put into their top. What most shops want to do is remove the canvas from the frame to install a new window, that is why it gets so expensive (labor). Knowing what I know now, I would consider finding a shop that would agree to replace the window
while the top is still on the frame. Like I mentioned, removing the top from the car was actually very easy.
If you are part of your local PCA, ask around to find top shop that has experience with Porsches.