UNION SELECT after LEFT JOIN and JOIN
Using Union and ORDER BY:

Be explicit with results form first query:

(SELECT DISTINCT rep.name, rep.date as rdate
FROM `reports_new` rep
LEFT JOIN `repuser_new` ru
ON ru.`id_rep` = rep.`id`
JOIN `clients` c
ON c.`id` = rep.`id_client`
WHERE ru.`id_user` = '1' and rep.`name` LIKE 'urgent')

UNION
(SELECT username,rdate FROM users)


ORDER BY rdate DESC


I assumed the second table had a date column. Otherwise and explicit value could be used.