I've currently got the Witcher 3 main game on Steam and I'd like to install the two DLCs. Is it possible to purchase (legal) activation keys via a different website (given that games are often cheaper outside Steam) and to enter them via the "Activate a Product on Steam" box? I know that this is easy to do with standalone games, but I'd like to check that it works with DLCs as well. Thanks!
Edit: I see that most websites sell GOG keys, which probably aren't compatible with Steam. Could I buy the GOTY edition on GOG and transfer the saves?