Don't think so steam is for steam and gog is for gog,steam has some drm probably and you then can't connect it to gogs dlc,you should have bough the game here on gog and then the rest of the dlc or on steam but it probably doesn't have a discount now on steam that you are asking this.

You cannot purchase DLC on GOG unless you already own (or are purchasing as part of the same order) the base game on GOG. You also cannot redeem a gift code for a DLC unless you have the base game in your library. Sorry.