Hi
You need to use also the badi ME_PROCESS_PO_CUST, method POST. Write something similar to:
DATA: ls_mepoitem TYPE mepoitem,
ls_mepoitemt TYPE TABLE OF mepoitem,
ls_header TYPE mepoheader.
DATA : i_items TYPE purchase_order_items,
i_header TYPE mepoheader,
l_single TYPE mepoheader.
i_header = im_header->get_data( ).
....
i_header-zzfield = 'VALUE'.
......
ls_header = im_header->get_data( ).
I hope this helps you
Regards
Eduardo