This is not an official response, but it IS one from someone who has attempted to do what you have done before multiple times.
1. Do you want a Steam copy of the game, or do you want your copies to be visible and launched from Steam?
If you want a Steam copy, then you have to buy it separately. Steam and Telltale are independent distribution services. Telltale gives you discs when you buy from them, and Steam usually has better sales. It's ultimately up to you wether you want the game (and nothing else) really, really cheaply, or if you want the bonus discs and other goodies that Telltale offers as a package when you buy from their store (even the standard edition comes with a bonus disc on most of their titles).
If you just want the games you have already purchased to be launchable from Steam-
Steam has provided a good tutorial here.
Hope this helps.