Solution : https://service.sap.com/sap/support/notes/669370 (SAP Service marketplace login required)
Key words :
select short text 'determine user-defined fields, changing user-defined fields, transfer user-defined fields, automatically created comments relating, delete automatically created comments, create function module ibapi_z_get_appends_of_struc, create includes libapi_cf01, function group ibapi_z, 'tables' tab page, function group ibapi_c
Related Notes :
| 739377 | BAPI_ALM_ORDER_MAINTAIN: Append fields not transferred |
| 708508 | Downgrade BAPI_ALM_ORDER_MAINTAIN -> errors |