Steam support representatives told me that I cannot register the game though Steam, and it says so in their FAQ as well. (I think it is because RSV2 keys only have 19 characters, and typical Steam keys have 20 or 25 characters. I think this is why it is incompatible.) I had bought it through Direct2Drive.
However, I would like to earn Steam achievements in Rainbow Six Vegas 2. I am able to get the Steam overlay by choosing "add a non-steam game" (as the Steam support instructed me to do). I am not sure though if it is possible to earn achievements; and if it is possible, I need to know how to do it.
In short, is it possible to earn Steam achievements in Rainbow Six Vegas 2? If so, how do I set up Rainbow Six Vegas 2 to earn achievements?