You have to include this code in the function module EXIT_SAPLRRS0_001.
CASE I_VNAM.
WHEN 'name of the variable'.
your code.
ENDCASE.
OR
There should be some statement in the this FM to call your function module.
Example:
WHEN others. "When it is none of the variables mentioned in the case statement.
IF I_VNAM IS NOT INITIAL.
IF sy-subrc EQ 0.
CALL FUNCTION "name of your function module" "This is where the function call is made to your FM
EXPORTING
I_STEP = I_STEP
TABLES
I_SOURCE = E_T_RANGE
EXCEPTIONS
NOT_FOUND = 1
OTHERS = 2.
IF SY-SUBRC <> 0.
EXIT.
ENDIF.