I already own DS2 on steam, could this Complete pack DLC work seamlessly with my existing copy?
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.
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.