Ok, only now i've found this:

https://www.slitherine.com/forum/viewto ... 58&t=93585

I suppose the answer is "buy it, redeem you steam key and register it in steam"

Hello, i'm not sure about something. I own Warhammer 40K: Sanctus Reach, the base game, in my Steam library. Can i buy the DLCs (digital version) on Slitherine site and add them to my Steam Library? Or use them with my Steam version of the game? If yes, how? (sorry for my poor english, i'm not a nat...