According to the list of supported retail games on Steam, you can not activate The Sims 3 with Steam, if you have purchased it from a retail store.
Some games are accepted, some games are not. As it is, I have found a few forum posts, complaining about how unreliable Sims 3 keys can be, while finding sources for this answer. One user reported not needing a key to play, but not being able to activate their key with the EA, themselves. This may have something to do with it; Valve would be hesitant to honour a CD key where even the developers appear to have issues identifying legitimacy.
This is also confirmed on the Steam forums, and on Yahoo! Answers.