Add message in BAPIRET2 table

Quick way to add message from system variables :

  APPEND VALUE bapiret2( type       = sy-msgty
                         id         = sy-msgid
                         number     = sy-msgno
                         message_v1 = sy-msgv1
                         message_v2 = sy-msgv2
                         message_v3 = sy-msgv3
                         message_v4 = sy-msgv4 ) TO lt_bapiret2.

Via dedicated form :

FORM add_message      USING pw_type TYPE sy-msgty
                            pw_id   TYPE sy-msgid
                            pw_num  TYPE sy-msgno
                            pw_v1   "TYPE sy-msgv1
                            pw_v2   "TYPE sy-msgv2
                            pw_v3   "TYPE sy-msgv3
                            pw_v4.  "TYPE sy-msgv4.

  DATA ls_bapiret2 TYPE bapiret2.

  ls_bapiret2-type       = pw_type .
  ls_bapiret2-id         = pw_id   .
  ls_bapiret2-number     = pw_num  .
  ls_bapiret2-message_v1 = pw_v1   .
  ls_bapiret2-message_v2 = pw_v2   .
  ls_bapiret2-message_v3 = pw_v3   .
  ls_bapiret2-message_v4 = pw_v4   .

  IF ls_bapiret2-type IS INITIAL.
    ls_bapiret2-type = 'S'.
  ENDIF.

  IF ls_bapiret2-id IS INITIAL.
    ls_bapiret2-id     = '0H'.
  ENDIF.

  IF ls_bapiret2-number IS INITIAL.
    ls_bapiret2-number = '000'.
  ENDIF.


  MESSAGE    ID ls_bapiret2-id
           TYPE ls_bapiret2-type
         NUMBER ls_bapiret2-number
           WITH ls_bapiret2-message_v1
                ls_bapiret2-message_v2
                ls_bapiret2-message_v3
                ls_bapiret2-message_v4
           INTO ls_bapiret2-message.

  APPEND ls_bapiret2 TO gi_user_message.

ENDFORM.

Check also post : Message : synthax templates