Yes. TSLPatcher can be set by the author to either automatically find the game's install location via a Registry lookup, or ask the user to find it manually. In the former case, it will fail to find a Steam install (because it stores its data in a different key) and fall back to a manual folder browser. So in either case you'll end up having to tell it where your game is. Once you do that it will work just like with the CD version.