How do I associate a Steam game with a separate, non-Steam install?

Solution 1:

AS far as I am aware, the Steam versions are different. Your best bet is to reinstall through Steam.

Solution 2:

A quick search yields this. In particular, the last answer might help you:

As far as what I'm asking about, it isn't the account I'm trying to associate, it's the software. I'm wondering if there is a way to get the Steam program to acknowledge my copy of STO that was not installed through Steam short of the 'Add Non-Steam Game' option or reinstalling via Steam, since I also have it through Steam now.

You can start the install in steam, then stop the download, close steam completely, copy or move the STO folders into the steam directory. Finally launch steam and validate files for STO, steam should recognise everything, it might download a file or 2 but it won't be much.

Thats what I did for the steam version of Fallout 3 goty and everything went smoothly (had the base game only and bought the package on sale for a few bucks, copied the FO3 files to steam and verified, saved myself a few hours download and have all the usual steam features as a bonus).

I suppose it can't hurt to try.