iBook opf won't add to library

I have moved some iBooks from Mac to PC in a container with the extension epub (but it is not an epub book in the normal sense.
The relevant contained file involved is 'Content.opf'.
I can open that in Calibre Viewer and it is perfect and I can print a pdf from it(albeit missing the TOC).
I cannot add the opf (container) book to the Calibre library so that I might convert it to true epub so that I can use an epub reader with annotation.
Any advice please?