@jecilop,
the difference is if you are willing to lift calibre's skirt and meddle with the structure, you can *probably* trick calibre into linking a new format.
But this thread, and Kovid's comment here, relates to trying to use check/restore library to
add a new book record.
Which is much more hopeless
unless you get all the details perfect. And creating an OPF yourself is horribly wasteful.