If you are looking for a way to have a hard-coded value, why not put it in the select like this (with or without the distinct depending on your data):
INSERT INTO GOREMAL(GOREMAL_EMAL_CODE, GOREMAL_EMAIL_ADDRESS)
SELECT DISTINCT '20', UOS_PIDM_ID(SIRASGN_PIDM) || '@sharjah.ac.ae'
FROM SIRASGN
WHERE SIRASGN_TERM_CODE = '201620'
AND NOT EXISTS
(SELECT GOREMAL_PIDM
FROM GOREMAL
WHERE GOREMAL_EMAL_CODE = 20
AND GOREMAL_PIDM = SIRASGN_PIDM);