Can you attach a previously bought copy to steam?

Hello everyone, I was hoping if someone could tell me whether or not it's possible to attach a previously purchased copy of the game to my steam account? It would be so much more handy to have it in there along with all of my other games.

 

Thank you, that was not quite what i needed, but i did figure it out. I had not checked the email this account is with in a long time and didn't realize i was sent a key a long time ago.