http://www.newportconvertible.com
He'll convert anything...for a price. As you can see he's done several four door cars (including a BMW 750iL, featured in MotorTrend, I think) The transition from sedan to 4 door convertible for the 750iL was a $21,000 modification. It made the car heavier, and thus slower, for body reinforcement.
I'm sure if he can do convertible tops he can do a rag-sunroof. Email them for more information.
As far as cars with frames around the windows is concerned, the Cutlass Supreme coupe (from which my convertible is built) started life in coupe form, and had part of the frame taken off the window. Hence the reason for the "basket handle" or "hoop" or "targa bar" across the top. The company that did the Cutlass Supreme convertibles was called Cars and Concepts, but GM mostly uses ASC (American Sunroof Company). Not that this is a GM board! LOL
It's possible to do anything, but structural rigidity and damage caused by torsional flex come into focus when chopping on cars.
Adding a sliding cloth sunroof would be cool, especially if it extended back for the rear passengers as well. However, with the cage that's formed by the B-pillar, it might be a little difficult.