MIRO details

Do someone Know how can I change the field ZUONR and BKTXT that are placed on the "Details field" in tx MIRO???? I´m trying to avoid the use of a BAPI for posting Invoces, I'm using a simple LSMW but those fields "asignación & header text" I would like to fill it with a user exit. I hope that someone can help me, thanks.