Buying the DLC pack on GOG will not provide you with a Steam key to unlock that content on Steam, GOG and Steam are separate companies and platforms.

It might be possible to buy the DLC on GOG then copy the files into your Steam folder, but getting the game to recognise them might require a kludge. Even if you do find a way, Steam would likely consider this a violation of their service, so you'd need to play in off-line mode at the least.

Overall, I'd suggest just buying the game again here if you really want to play with the DLC. It's probably not worth the effort and risk.

Thanks for the answer Lemmus. I was in the same place - with wanting the DLC but having a Steam version.

You can't buy/redeem the DLC pack on GOG, if you don't already have the base game here. It's all or nothing.