Manipulating 810 EDI AP Invoice Inbound file data

We currently have a EDI810 translated flat file sitting on the server, which has values for INVOICE_DATE and other such fields, which we don't want. We would not like some of these field values  to go into interface tables, even though they  are in the translated flat file. Is there a way to manipulate or default them to NULL while getting inserted into the interface tables. We run the "IN: Invoice (810/INVOIC)" after we have the translated file on the server. Once this is run it automatically kicks off the Payables Open Interface program. We do not know how to manipulate the values in the file either before inserting into interface tables or after inserting into interface tables. We have explore the "Interface File Definition" form for Transaction 'INI' under Setup, but could not come up with any solution.