[esperent]

I got curious after writing my comment and decided I should research it. What I found was a couple of people on reddit saying it was possible, but getting a ton of downvotes and replies saying that they're wrong, that it's no longer possible. At the very least it looked fairly convoluted, and possibly you need to also own the game on steam to download mods for non steam (pirated or otherwise) versions of the game.

I don't have an oar in this fight - my comment about pirated games was mostly tongue in cheek - but I am glad the main mod site isn't tied to owning the game on one particular store.