Hello,

Is it possible to purchase the game here, and add it to steam?

Nope. You won't get a redeemable Steam key. GOG is GOG and not a key reseller.

Not as a Steam key.

You can add the game via "Add a Non-Steam Game" function in Steam. The games here are otherwise fully independent of the Steam service. They're designed to be installed the old-fashioned way so that you have more control over the installation.

Edit: Dem ninjas.