How to insert a code for a function module into a Customer Exit Variable?

Question

I have two Key Figures viz., Net Prchs Rtl, and Net Prchs Unt. Both these Key figures have This Week (TW) and Last Week (LW). There is a variable for This week but there is no variable defined for Last week.
I need to get data in the column LW (Last Week) for both the key figures.
In function module EXIT_SAPLRRS0_001 one of the functions I have is:
Get the previous Fiscal Week
          CALL FUNCTION 'DATE_TO_PERIOD_CONVERT'
I need to create a new variable to get values in the Last week column for different key figures and use function “'DATE_TO_PERIOD_CONVERT'” in that variable.
Can anyone please explain me the steps as to how to use a function module in a variable so that when the variable is used in a key figure it shows the output.
In other words what I want to know is after creating a Customer exit variable of type Characteristic value how do I refer the above mentioned function moduel and insert the code for the function module into the Customer exit variable that I created.
Thank you.
TR.

Answer 1

Hi Wond,
Thanks a lot for your reply. I understand what you mean but I have never done this before so can you please explain it in a detailed manner. I have the following code:
Get the previous Fiscal Week
          CALL FUNCTION 'DATE_TO_PERIOD_CONVERT'
            EXPORTING
              i_date         = ld_date
              i_periv        = lc_periv
            IMPORTING
              e_buper        = ln_poper
              e_gjahr        = ln_bdatj
            EXCEPTIONS
              input_false    = 1
              t009_notfound  = 2
              t009b_notfound = 3
              OTHERS         = 4.
          IF sy-subrc <> 0.
            MESSAGE ID sy-msgid TYPE sy-msgty NUMBER sy-msgno
            WITH sy-msgv1 sy-msgv2 sy-msgv3 sy-msgv4.
          ELSE.
            lc_poper = ln_poper.
            CONCATENATE ln_bdatj lc_poper+1(2) INTO wa_e_t_range-low.
            wa_e_t_range-sign = 'I'.
            wa_e_t_range-opt = 'EQ'.
            APPEND wa_e_t_range TO e_t_range.
          ENDIF.
        ENDIF.
      ENDIF.
    ENDIF.
I believe I should now use the above code in my customer exit variable. If that's right can you please explain me the steps as to how should I do this so that the variable gets populated.
Thank you.
Regards,
TR.