You can create field name in run time and assign the created name in your field-symbols by statement "ASSIGN ... TO ...".

Example

DATA: name(20),
      lv_check(1).

DATA: BEGIN OF itab OCCURS 0,
        fld1(4),
        fld2(4),
      END OF itab.
FIELD-SYMBOLS <fs>.
...
...
LOOP AT itab.
  IF lv_check = ’X’.
    name = ’ITAB-FLD1’.
  ELSE.
    name = ’ITAB-FLD2’.
  ENDIF.
  ASSIGN (name) TO <fs>.
  ...
  ...
ENDLOOP.