Convert to steam key

I purchased this pretty much in the first wave --- still doing the 'download, find key, plug it in' thing.
How do I convert the game to a Steam available version?
Thanks in advance. I've looked all over on how to do this and I cant find it..... but, I am asking after looking....
How do I convert the game to a Steam available version?
Thanks in advance. I've looked all over on how to do this and I cant find it..... but, I am asking after looking....