Just add 'Empty books' (right click the Add Book Icon). You don't need any actual file (Calibre will create a 'cover' and a backup of the metadata in the title folder as normal)
If you have a spreadsheet? Look
hard 
at the
Import List Plugin.
Add from ISBN is how I did min with a barcode scanner (that way also fetches a cover if available, just like a regular add)
I als tag mine <In Paper Library>