This is one of our contemplated ideas. (http://thpatch.net/wiki/Development_notes/Future_ideas#Steam_Integration)
That said, all you need to do is go to "Add non-steam game", and look at the thcrap shortcut and enter in those details.
And now with Screenshots.
1. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Add%20Nonsteam%20game.png) 2. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Browse.png) 3. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Select%20Game.png)
4. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Properties.png) 5. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Start%20%26%20Target.png)
6. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Fill%20them%20in.png)
7. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Play.png)
Lemme know how it goes!
Update: I hear some have trouble doing it this way, for a sure fire way to have it work, select the unpatched game first, then go to properties, then click view all files in the dropdown and then select the patched shortcut. And of course, with pictures. ( Shrunk down too, it was getting huge! )
1. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Add%20Nonsteam%20game.png) 2. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Browse.png) 3. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Select%20Game.png)
4. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Select%20properties.png) 5. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Change.png) 6. (https://dl.dropboxusercontent.com/u/33350662/thpatch/All%20files.png)
7. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Select%20patch.png)
8. (https://dl.dropboxusercontent.com/u/33350662/thpatch/Enjoy.png)