1. Open Steam
2. Click on Games < Add a Non-Steam game to My Library
3. Wait for the list to complete
4. Check if your game is listed there, if not, you have to Browse to the game.
5. After you've browsed to the game, it should be added & selected to the game list, and you only need to click "Add selected Programs".
6. Now, for it to work, you need to launch it through Steam. Meaning that if you try to launch the game with an old desktop shortcut, it won't work. So, you need to go to "My Library" on Steam. Find the game, and right-click it < Create Desktop Shortcut.
7. Now, you should have a shortcut on your desktop, but without an icon. It doesn't matter, but it's ugly ;3 To fix that, right-click the shortcut, then "Properties". Go to the "Web Document" tab, and click on "Change Icon". Browse to the same path you did when adding the game to Steam, and it will get the same icon as that file. Click "OK", and you're done!
I've only tested this with Dragon Age II, and the overlay works there. But Steam says "Some detailed information on [GAME NAME] is unavailable because it's a non-Steam mod or shortcut. Steam will still manage launching the game for you and in most cases the in-game overlay will be available."
