@elena @j3j5 To be fair, you could add boks from any store already, you just had to convert them to a supported format (which, granted, is hard if the books you try to convert come with DRM).
Calibre (a neat little library management software) does many thing, and one of them is converting ebooks. And it supports outputting ebooks in the two format supported by kindles (azw3 and mobi)