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)

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.

There were quite a few documents and help pointing out how to do this but none putting it all together in one document and setting you on your way. You can say that no more. Enjoy!!!

It is sometimes necessary to link an RFC destination to a logical system (when doing synchronous calls between systems and you want to use the logical system).... Table entries are in TBLSYSDEST.

 Use transaction BD97 to do the linking.

Modularization Techniques

Processing blocks that are called from ABAP programs:

1. Subroutines


2. Function modules


3. Methods
   
   Procedures
   Procedures contain a set of statements, and are called from other ABAP programs.
   The processing blocks that you call from ABAP programs are called procedures
   You define procedures in ABAP programs. When the program is generated, they remain as standalone modules. You can call procedures in the program in which they are defined, or from external programs. Procedures have an interface for passing data, and can also contain local data.
   
   ABAP contains the following kinds of procedures:

Source Code Patterns.

In the ABAP editor there is the functionality to insert code into your program using what is called a pattern. You use this functionality each time you insert a function call for example. This menu option is reached via the menu path SE38->Edit->Pattern:
 

Calling An External Program From SAP.

Operating system programs and other application programs can be called from SAP in one of four ways.

The first two methods depend on the location of the application program – whether it resides on the local PC, or the application server. Both are easy to implement. The third way, using ABAP objects relies on the application being called being an OLE server. The fourth I regard as a kind of back door....

An OLE server uses object technology for sharing information and services across process and machine boundaries.

Programming a BAPI consists of 6 major tasks:

1. Defining BAPI Data structures in SE11
2. Program a RFC enabled BAPI function module for each method
3. Create a Business object for the BAPI in the BOR
4. Documentation of the BAPI
5. Generate ALE interface for asynchronous BAPIs
6. Generate and release

Tip: You can use the BAPI explorer to get a step by step instruction / checklist in how to create a BAPI. In the BAPI explorer select the Project tab.

Comprehensive list of function modules and descriptions of what they do.....

You can use the DCOM connector or the Java Connector to connect to an SAP system if you have access to a programming language like VB, C++ or Java. If you don't and a client of  yours needs to extract data from an SAP system to a flat file then here is the procedure to accomplish it. We make use of a DOS batch file which will call the SAP standard STARTRFC executable to attach to the SAP machine.