You won't even be able to buy the expansion here if you don't already own (or purchase at the same time) the base game here -- the one exception being if you buy it as a gift code...but, even if you did, and sent the gift e-mail to yourself, I don't know if you'd be able to redeem the game with your account without owning the base game. EDIT (for posterity): You would not.

Even if you did figure out some arcane way, though, there'd be no way to know if it'd work except by trying it -- and if it wasn't compatible with your non-GOG base game, you'd be out the money, as they obviously don't support games bought elsewhere, and can't guarantee that the GOG-made installers for the DLC would work with them.