Solution : https://service.sap.com/sap/support/notes/739377 (SAP Service marketplace login required)
Key words :
fill customer-specific fields, fill append structure fields, it_methods-method = 'create', structure ci_aufk, structure caufvtab, create case, badi ibapi_alm_ord_modify, order header, bapi bapi_alm_order_maintain, table aufk
Related Notes :
| 669370 | BAPI_ALM_ORDER_MAINTAIN: BAdI not for append structures |