You can include the Top Bosses ID in each row
e.g.
WITH TopBosses(ID,NAME,BOSS_ID,TOP_ID)
AS
(
SELECT ID,NAME,NULL AS BOSS_ID,ID AS TOP_ID
FROM EMPLOYEE
WHERE BOSS_ID IS NULL
UNION ALL
SELECT EMPLOYEE.ID,EMPLOYEE.NAME,EMPLOYEE.BOSS_ID,TopBosses.TOP_ID
FROM EMPLOYEE
INNER JOIN TopBosses
ON EMPLOYEE.BOSS_ID = TopBosses.ID
)
SELECT ID,NAME,BOSS_ID,TOP_ID
FROM TopBosses