i guess its already been asked but somehow i seem to be unable to find an answer that helps :

can i get the drm-free/gog version and use it with my steam base game or do i have to spend another 5 bucks so i have a copyright on it ? o_0

thx in advance, spent 40 min by now on this topic

If you have the steam Base game then you need to get the dlc on steam.

You can also get it at Humblebundle.com to get both.(Guess the DRM free installer might not work without the DRM free basegame still thought...)