This is perhaps an odd or silly question, and it needs a bit of elaboration.
I have two copies of a game, one from Steam, and one from the developer's website. This is because I purchased the game before it became part of a Humble Bundle. In the case of Arkham Asylum, there is a mac version of the game, but Steam does not carry it. They DO carry Arkham City's mac version, though. Darned if I know why.
I've looked in the package content of Steam games before, and they come with extra files, namely on how to handle Steam integration. I'm going to try to replicate these files into my mac copy of the game, with all the symlinks. I want to see if this will allow Steam to track my data in the game, mostly, such as time played and achievements earned. If this is not possible, I won't be heartbroken. I know this is a stretch at best. I come here because I've scoured the internet in search of an answer to this, and this is the last place I can think of.
Has this been done before? If so, to what degree of success? Am I crazy to think that this replication might work?