From my understanding, based off the Letter from the Producer, you have to purchase a Steam version of the expansion to have it work.

I've been checking Steam every time I get on my computer to see if there's any information on that end. So far, I haven't seen anything - which doesn't bode well for a preorder that's supposed to be available soon.

I imagine it will be available to preorder on Steam before the game is released, not sure about today, but I worry about not being able to get the bonus items and early access if we have to go through Steam. If that's the case, I'd be highly disappointed in SE as a physical item through their store is one thing, a digital shouldn't matter.