Although it looks like an ePub (which is a single zip file), it's actually a folder that's had a 'package' bit set so that it looks like a file in the Mac OS X Finder.
This probably means they're from Apple iBooks.
You can convert them to proper zipped ePubs using my handy AppleScript. Just download from
https://www.mobileread.com/forums/sh...ad.php?t=55681
Once they are actually zip files, you'll be able to import them to calobre. Although if they are from iBooks, they'll probably have DRM making them storable but not readable in calibre.