You could check some suitable BAdI of reference note 1156325 - BAdIs in the Logistics Invoice Verification environment. But those BAdI and exits like EXIT_SAPLMRMP_010 (*) don't allow modification of such field, copied from PO, there is an option to add some customer fields in the BAdI list.
So I should suggest you to analyze the report to find some implicit enhancement point during filling of the internal table.
Regards,
Raymond
(*) Header is import type (and even the table for item is a local copy of the work internal table of main program, so any change would be discarded (perform a where-used search)