LOOP AT ft_out INTO fl_out.
...
SELECT SINGLE belnr INTO f_belnr FROM bkpf
   WHERE bukrs = fl_out-bukrs AND stblg = fl_out-belnr AND stjah = fl_out-gjahr.
...
APPEND it_out.
ENDLOOP.

LOOP AT ft_out INTO fl_out.
...
SELECT SINGLE stblg INTO f_belnr FROM bkpf
   WHERE bukrs = fl_out-bukrs AND belnr = fl_out-belnr AND gjahr = fl_out-gjahr.
...
APPEND it_out.
ENDLOOP.

TYPES: BEGIN OF fs_out,
    belnr LIKE bkpf-belnr,
    mblnr LIKE mseg-mblnr,
    sdfak LIKE vbrk-vbeln,
    sddel LIKE likp-vbeln,
    bukrs LIKE bkpf-bukrs,
   gjahr TYPE gjahr,
END OF fs_out.
DATA: ft_out TYPE STANDARD TABLE OF fs_out,
          fl_out LIKE LINE OF ft_out.

" Заполняется данными
" Каждая строчка уникальная
" В разных ситуациях поля документов могут быть пустыми

LOOP AT ft_out INTO fl_out.
CLEAR it_out.
<...>
IF fl_out-belnr IS NOT INITIAL.
  SELECT SINGLE belnr INTO f_belnr FROM bkpf
     WHERE bukrs = fl_out-bukrs AND stblg = fl_out-belnr AND stjah = fl_out-gjahr.
  IF sy-subrc <> 0.
    it_out-stats = 'E'.
  ELSE.
    it_out-stats = 'S'.
  ENDIF.
ELSEIF fl_out-mblnr IS NOT INITIAL.
... " MSEG
ELSEIF fl_out-sdfak IS NOT INITIAL.
... " Своя логика
ELSEIF fl_out-sddel IS NOT INITIAL.
... " Своя логика
ENDIF.
APPEND it_out.
ENDLOOP.

Вот так?
SELECT SINGLE bk2~belnr INTO f_belnr FROM bkpf AS bk1
  INNER JOIN bkpf AS bk2 ON bk1~bukrs = bk2~bukrs AND bk1~belnr = bk2~stblg AND bk1~gjahr = bk2~stjah
  WHERE bk1~bukrs = it_out-bukrs AND bk1~belnr = it_out-fidoc AND bk1~gjahr = it_out-gjahr.

" или

    SELECT * INTO ft_bkpf
             FROM bkpf
             UP TO 1 ROWS
      WHERE bukrs = it_out-bukrs AND stblg = it_out-fidoc AND stjah = it_out-gjahr.
    ENDSELECT.

IF fl_out-belnr IS NOT INITIAL.
  SELECT SINGLE belnr INTO f_belnr FROM bkpf
     WHERE bukrs = fl_out-bukrs AND stblg = fl_out-belnr AND stjah = fl_out-gjahr.
  IF sy-subrc <> 0.
    it_out-stats = 'E'.
  ELSE.
    it_out-stats = 'S'.
  ENDIF.