How to import from calibre to another calibre library

I have access to my friens's Calibre library where all books are already with metadata and formatted for Calibre. If I just copy them into my CalibreLibrary folder, they are not, of course, added automatically into my Calibre database (and are reported as "extras" when checking the library). Do I need to import them manually (by using only the mobi or other book format) or can it be done in batch, automatically?

Thanks a lot,
Martin