If the library is a valid calibre library, and it is stored in the local filesystem, then you don't have to add it at all. Just open it. To avoid problems with too long paths it is best to store libraries near the root of a drive.
To check if it is a valid library, verify that the file metadata.db is present in the top folder of the library. That is the database with all metadata. Also you have to be certain that you haven't manually added, removed or renamed any files in the calibre library. If you have then the library most likely is corrupt as far as calibre is concerned. Only calibre may update the contents of the calibre library.
So what you do is copy the calibre libraries over to the local harddrive, separate folders for each, of course. And open them.
http://manual.calibre-ebook.com/gui.html#library