I've gotten Zipped files right here on this site (where the public domain books are offered after they are carefully prepared and uploaded by thoughtful members). I download the ones under "Epub". Zipped. Inside, no epub files. I let Calibre look at the zipped files anyhow and it magically sorts things out and gives me epubs of the files in the zipped version
So it might work for you, too, with the Kobo books.