To do so you first need to ensure that the following 5 tables are in synch.

1) VBAP: In your append structure you specify your ZZFIELD with the data type as needed

2) BAPE_VBAP: In the append structure here also add the ZZFIELD with the data type as needed with limitations. No decimals. Try and stick to char characters

3) BAPE_VBAPX: In the append structure add the field ZZFIELD of type BAPIUPDATE

(NOTE: (2) and (3) must have the same number of fields in the same order)

4) VBAPKOZ: In the append structure here also add the ZZFIELD with the data type as needed with limitations. No decimals. Try and stick to char characters

5) VBAPKOZX: In the append structure add the field ZZFIELD of type BAPIUPDATE

(NOTE: (4) and (5) must have the same number of fields in the same order)

(Note: A great tip from Hendrik Maas: Use Data element CHAR1 instead of BAPIUPDATE as it will not work for BAPI_SALESDOCUMENT_CHANGE)

Similarly do for VBAK, BAPE_VBAK, BAPE_VBAKX, VBAKKOZ and VBAKKOZX.

Next we get to the code to fill in the structure EXTENSIONIN
I will demonstrate how to call the create sales order BAPI with custom fields.

* Local definitions
  DATA: wa_extensionin TYPE bapiparex,
        wa_bape_vbap   TYPE bape_vbap,
        wa_bape_vbapx  TYPE bape_vbapx,
        wa_bape_vbak   TYPE bape_vbak,
        wa_bape_vbakx  TYPE bape_vbakx,
        lv_posnr       TYPE posnr.

* Processing the header extension
  CLEAR wa_bape_vbak.
  wa_bape_vbak-ZZFIELD     = ‘HDRTEST’.
  wa_extensionin-structure = ‘BAPE_VBAK’.
  wa_extensionin+30(960)   = wa_bape_vbak.
  append wa_extensionin to lt_extensionin.
  clear wa_extensionin.

* Processing the line extension
LOOP AT line_items INTO wa_lineitems.

  ADD 10 TO lv_posnr.   
  CLEAR wa_bape_vbap.
  wa_bape_vbap-ZZFIELD     = ‘TEST’.
  wa_extensionin-structure = ‘BAPE_VBAP’.
  wa_bape_vbap-posnr       = lv_posnr.
  wa_extensionin+30(960)   = wa_bape_vbap.
  append wa_extensionin to lt_extensionin.
  clear wa_extensionin.

  CLEAR wa_bape_vbapx.
  wa_bape_vbapx-ZZFIELD    = ‘X’.
  wa_extensionin-structure = ‘BAPE_VBAPX’.
  wa_bape_vbapx-posnr      = lv_posnr.
  wa_extensionin+30(960)   = wa_bape_vbapx.
  append wa_extensionin to lt_extensionin.
  clear wa_extensionin.

ENDLOOP.

* Then the call to the BAPI
      CALL FUNCTION ‘BAPI_SALESORDER_CREATEFROMDAT2’
        EXPORTING
          order_header_in  = ls_order_header_in
          order_header_inx = ls_order_header_inx
        IMPORTING
          salesdocument    = lv_salesdocument
        TABLES
          return           = lt_ret2
          order_items_in   = lt_order_items_in
          order_items_inx  = lt_order_items_inx
          order_partners   = lt_order_partners
          order_keys       = lt_order_keys
          extensionin      = lt_extensionin.

Note: If you have a need to force a different Business Object type or wish to see the extension return fields then use the function SD_SALESDOCUMENT_CREATE instead.

      CALL FUNCTION ‘SD_SALESDOCUMENT_CREATE’
        EXPORTING
          sales_header_in     = ls_order_header_in
          sales_header_inx    = ls_order_header_inx
          business_object     = ‘BUS2032’
        IMPORTING
          salesdocument_ex    = lv_salesdocument
          sales_header_out    = lv_sales_header_out
          sales_header_status = lv_sales_header_status
        TABLES
          return              = lt_ret2
          sales_items_in      = lt_order_items_in
          sales_items_inx     = lt_order_items_inx
          sales_partners      = lt_order_partners
          sales_keys          = lt_order_keys
          extensionin         = lt_extensionin
          incomplete_log      = lt_incomplete_log
          extensionex         = lt_extensionex.