They allow adding alternate repositories, but the ones I've come across have all also only allowed FOSS apps. I wonder if it's a policy any repo has to follow or if it's possible for someone to create a non-FOSS repo too.
Even when you can use a non-free repo, that still leaves the question of payment. Because most who distribute nonfree apps, want money first.
I doubt this is compatible with F-Droids architecture.
