I tried the double extension on one book. Maybe my choice wasn't the wisest because it adds a lot of room for errors since it was originally from Amazon (so .azw -> .epub -> .kepub.epub).
I did was davidfor recommended in the comment #15 so the eReader can display the cover. Then I manually changed the extension to add .kepub and drag-and-dropped the file into (onto???) the Mini.
So far, the cover, the pages per chapter, the estimated time are working. The highlights are not working (as expected). For remembering the last page read, it's working if I read only this book. If I open other books, when coming back to the kepub.epub one I will be brought back to the first page of the chapter.
I'm glad I gave it a try, though
