Вам нужен Союз, но так как TOP не разрешен в Set-Select, вы должны вложить его в Производную таблицу:
SELECT *
FROM
(
SELECT TOP 1000 * FROM XX
ORDER BY REVENUE DESC
) as dt
UNION
-- UNION ALL is more efficient, but a user from the custom list who is
-- also in the top 1000 would be returned twice
SELECT * FROM XX
WHERE USER_ID IN (CUSTOM_LIST)