As of right now, you can enter the CD key from the games on this list
https://support.steampowered.com/kb_...7480-WUSF-3601 , and have it be a "Steam" game with achievements, downloads and updates and the like (I did this with Fallout:NV). You can add any game and launch it from inside Steam to keep your Steam overlay available (chat, friends list, etc), but it doesn't keep track of achievements or play time unless it's on the list.
Edit:
https://support.steampowered.com/kb_...2219-YDJV-5557 Here's the procedure for adding non-Steam games.