If I own Age of Wonders III on Steam, can I purchase and use the expansion pack(s) from GoG?

I seriously doubt that Steam would even allow it.

That's... some twisted logic you have there. >_>

The expansion is currently on sale here at GoG. I'm just trying to get a definitive answer before it goes back up in price. I'm also inquiring at the developer's website and Steam itself.

Not compatible! You'd need a Steam key to activate it and GOG does not sell these.

Asking for DLC from GOG to work on a game from Steam is like going to McDonalds and asking for Burger King onion rings with your value meal. If a game is already tied to Steam, so is its DLC. If GOG sold publisher license keys, you might be able to take it over to Steam and use it, but GOG doesn't do that.

Got it. I appreciate the quick and clear answers!