I'm trying to use the isbn number on books as the book barcode. however,
It's supposed to be just a number to uniquely identify each physical item, and usually doesn't have any inherent "meaning", like a biblio-level call number. If you really want to, you could of course construct your item-level barcode by using the biblio-level ISBN number and then just adding an item-level suffix to make it unique. But I can't really think why you would want to do that, as you would still need to print (or buy) and physically apply the barcodes. Plus I've seen many instances where completely identical copies of a given book (just a different print run, not a different edition) had different ISBNs. What most people do is print out (or buy) a big batch of barcode stickers, and apply them to the physical books at the same time as entering the item-level holding information.