@gamingonlinux they did it once for Destiny 2 if I'm not mistaken, when Bungie announced they were moving from Bnet to Steam.

you don't even need extra keys, just make it so that when you link your Bnet with Steam, you get the game on your Steam. but this seems like it's just a shady practice by ActiBlizz to get more money for users that want to play on Steam.