@AndrewT this more your area of expertise but as far as I'm aware any purchase made through the Paradox Store provides you with a Steam Key. So you'll be able to redeem and launch the game/expansion through Steam as you would almost any other PC game nowadays. I assume that if your Steam version of Age of Wonders: Planetfall appears in your Paradox Launcher V1 the Expansion should work all the same when launched through the Paradox Launcher.