How can I convert numerals into the corresponding text?

Use the Function Module SPELL_AMOUNT to convert the integer into text.

DATA v_int TYPE i VALUE '1000'.
DATA words LIKE SPELL.

CALL FUNCTION 'SPELL_AMOUNT'
EXPORTING
  AMOUNT          = v_int
  LANGUAGE        = SY-LANGU
IMPORTING
  IN_WORDS        = words
         .
WRITE words-word.