It is! I wanted to do it for my car, as the steel sunroof was the only available roof for the classic trim in 1998. My wife's E300 and her C36 both had the glass roof, and I really liked it.
However, it ain't cheap. You have to buy the glass roof, the headliner for it, and then swap the panels. My dealer priced out the components at about $1300. I called around to a bunch of wreckers, and not one had a wrecked W202, much less one with a glass roof. The dealer also thought that the headliner might be different, but after seeing the panel price, I stopped there. It does make some sense, as the glass roof has the interior sliding panel that the steel roof does not.
For $1300 or more plus the time and energy required to pull the headliner and swap the panels, I'll live with my steel roof.
Your arms might be longer than mine (able to reach into deeper pockets...