Updting the custom fields of PO Item

Hi I have added 1 field in the po item and po header throough secreen enhancement, now the requirment is whenever user enters any value in the header custom field, the same data should be updated to all the items...... tried to use

EXIT_SAPMM06E_018 which is working fine when I have only one item and it is working fine when user opens the item . my requirment is user will just add the data at header's custom field and will not open each and every item but the same data should be copied to all the items...tried to use

PROCESS_ITEM, check and post methods of badi.... in check method I am able to change standard fields

but not working for the custom fields, did not find a solution yet that's y posting again.... please suggest