Due to the difficulty of exactly matching the curvature of the roof, the only good way to install a sunroof in a 122 is to find one of the extremely rare (not officially imported to the US) 69 or 70 2 doors that has one as original equipment. It is not beyond practicality to transplant this roof into another 122 sedan, (2 or 4 door) assuming you can find a rusty or otherwise unrepairable donor car with a sunroof, which is not very likely.

Probably the best thing to do is forget about it---if you want a 122, you will have to accept the fact that it won't have a sunroof. If you really want a sunroof, 240 sedans had them, (usually on the GL models) and while they are not all that common, they aren't all that rare either, so your chances of finding one are fairly good. There never were factory sunroofs on 240 wagons. I've heard of a few that had aftermarket installations. Most 740s and 940s have a sunroof. No 1800s of any kind ever did.