Solution : https://service.sap.com/sap/support/notes/143580 (SAP Service marketplace login required)
Summary : 
      This SAP Note provides guidance on integrating customer-specific fields into SD tables VBAK, VBAP, and VBEP with SD BAPIs supporting the EXTENSIONIN parameter. For character-type fields, the EXTENSIONIN parameter should be used following BAPI programming guidelines or customer enhancement documents. Non-character types require referencing SAP Notes 700795 and 509898. The process involves defining fields in the INCLUDE structure CUTEST, appending these to BAPI structures using the APPEND technique, and managing data flow through internal communication structures to update SD tables appropriately when using BAPI.
Key words : 
bapisditm-itm_number  = '000020'      item number            bapisditmx-itm_number = '000020'      item number          + bapisditmx-updateflag = 'u'          updkz, bapivbeln-vbeln       = '0000004711'  document number            bapisdhd1x-updateflag = 'u'           updkz, data element/component typecfield2          char1cfield2           char1 3, order 4711 customer field vbap-cfield2, prerequisites solution technical information, character-type field types, structure                  valuepart1        1234567890123bape_vbap                  0000004711000020            xyzbape_vbapx                0000004711000020, chapter 'customer enhancement concepts, item vbkd fields, character-type types
Related Notes : 
       
| 1071365 | FAQ: New fields in R/3 system from EEW enhancements | 
| 1053817 | Distributing customer-specific fields from CRM to R/3 | 
| 763791 | BAPI_SALESORDER_SIMULATE delivery priority | 
| 700795 | Enhancement category of BAPI enhancement structures | 
| 550431 | FAQ: BAPI in sales - general questions | 
| 509898 | BAPI enhancement concept and Unicode | 
| 414020 | MASS: Customer fields are not mass-maintainable | 
| 356849 | BAPI problems during customer enhancements of VBKD | 
| 150242 | Including additional fields in interfaces | 
| 143602 | Incorrect processing EXTENSIONIN in SD BAPIs | 
| 93091 | Information about the BAPIs in Sales and Distribution |