I can't find one either, but yes it can support UDF's. That would handle it, but the issue is still what to use for the primary serial number. They are not a mfg, so the items arriving already have the sn created.
They would have to put something in that field at time of receipt. If you used the first 15 chars of the 18 digit string they need, duplicates would be a problem. You could potentially use the last 15 chars of the 18 digit string and chance of duplicates would be decreased, but they want to scan the barcode at receipt. Not all sn are 18 chars, so programming the scanner to truncate chars is not an option.