Torchlight 2 - Bought from Runic Games, unable to be added to steam

[Taiko]

What the title is basically saying; I want to take advantage of the Buy 4 deal, but I don't want to accidentally add it to my Steam library, and adding Torchlight 2 to my library is seeming to be impossible. Does anyone know if there's a way around this? I'd rather not get the same game twice over, awesome as it is.