Print cumulative field in table

Question

Hello,
Iu2019m just a beginner with Interactive Forms but maybe there is someone who could help me with the following issue:
In my first form I have to display some columns from the table VBFA (Sales Document Flow) One of the columns (RFWRT-  Reference value) contains the amount.  The cumulative value of this column must also displayed in the form. So Iu2019ll capable to import the table with table type VBFA_T into the form. If I drag the table on the layout I see the contents of the field RFWRT.
But my question is how could I include the cumulative field to the table?  Just describe the steps in more detail how to do this. Here there is a part of the interface code I already have:
DATA: VBFA              TYPE VBFA_T,
      ERROR_STRING      TYPE STRING,
      RETURN            LIKE BAPIRETURN1.
  SELECT *
    INTO WA_VBFA
    FROM VBFA
    WHERE VBELV =
    AND VBTYP_N = 'M'.
    APPEND WA_VBFA TO VBFA.
  ENDSELECT.
Now call the generated function module.
CALL FUNCTION FM_NAME
  EXPORTING
    /1BCDWB/DOCPARAMS  = FP_DOCPARAMS
    VBFA               = VBFA
   IMPORTING
     /1bcdwb/formoutput =
  EXCEPTIONS
    USAGE_ERROR        = 1
    SYSTEM_ERROR       = 2
    INTERNAL_ERROR     = 3
    OTHERS             = 4.
Kind regards,
Richard Meijn

Answer 1

If I open the interface and open the tree "Form Interface" and the "Import" level I see the parameter and the type name.
In my case Parameter name = VBFA and the Type Name = VBFA_T. The table type VBFA_T has a line type VBFA which is a clustered table. I'll could appends the structure with the cumulative field in SE11 but is it possible to do it on a nice way to create a new (Clustered) table ZVBFA with structure VBFA and append the desired field there?
I'm so far I know not able to add a (cumulative) field to a new created table/view because VBFA is a clustered table.
Kind regards,
Richard Meijn