Might be because I'm using Calibre 2.38 on Mac OSX...
Calibre added the .zip file to the library, but when I tried to open it in Calibre it launched the system archive utility. After reading
http://techie-buzz.com/helpdesk/how-...-archives.html I tried changing the file extension to .cbz and adding that the the library; opening that in Calibre launched Stuffit... I'll keep searching.
Comic Book Archive Creator is Windows only, unfortunately, but thanks for the idea.
I'll keep looking for a free creator. Thanks!