PetePl: I want to add this game to the steam library
so I'm asking you if I have to buy Darkest Dungeon a second time?
HereForTheBeer: Linko90: Yes, you will have to buy it on Steam.
Sorry!
HereForTheBeer: Yeaaaaah, don't do that.
There should be a way to add games so it shows up in your Steam client library (but not in your Steam store library). I don't use Steam and don't know myself how to do it, but there have been plenty of threads over the years detailing how it's done.
Try that before spending any money to buy a second copy that's not technically needed.
That's not adding it to your Steam library (no achievements, sync etc) which is what i understood the question was about it, hence the reply. Hope this clears things up!