Selling on an Amazon marketplace we do not route to? BookRouter can now build the upload files for you: a filled Listing Loader spreadsheet, a listing feed, or a delete file, priced in the currency you sell in.
The same page can attach ASINs in bulk from an Amazon Active Listings report, which is what gets items with no ISBN onto Amazon at all.
See Linking items to Amazon with an ASIN in the Help Center.