Power sunroofs "after the fact" are MUCH more common than people think. A lot of vehicles you'll see with power roofs actually had them installed later, but when professionally done, they look OEM.
There is at least one or two other people on other Lambda boards (Acadia & Enclave) that have mentioned having a sunroof installed on a vehicle without one, and it's a common practice elsewhere. The dealer does NOT actually do it, but has agreements with local specialty shops and installers who do the work daily. The same people, usually, who also do leather after the fact (also very common), heated seats (another common accessory, especially on vehicles with cloth or whatever, that just don't have them).
So, yes, it's common and usually the results are fantastic. Most of the aftermarket ones (that are made by the same companies who supply car makers with their OEM sunroofs), such as from ASC, Webasto, etc. Here's some of their sites:
http://www.inalfasunroofs.com/
http://www.webasto.us/am/en/am_auto_sunroofs.html
Here's a good sample of a regular sunroof installed in a Cadillac SRX, that only offers the full roof Ultraview from the factory. You can see how it is integrated, where the button is, etc.:
http://www.webastoshowroom.com/images/04cadillac_srx730.pdf
Find out who the shop is who does their installs, and check them out. Most are very well skilled and experienced, and should be more than willing to show you some of their work. These kind of installations are usually warranted for life or a very long time, too, and the end price isn't much more than what an original OEM roof would be--but with more features.