Perhaps you could try something like
SELECT *
FROM Parent.Table
WHERE Ref_Num In (SELECT Ref_Num
FROM Child.Table
WHERE Ref_Num ='234'
UNION
SELECT '4205' As Ref_Num
FROM SYSIBM.SYSDUMMY1)
AND Asset_Type IN ('PUMPS','COLL')
Note that SYSDUMMY1 is an EBCDIC table. If you need ASCII, use SYSDUMMYA. If you need Unicode, use SYSDUMMYU.
Share and enjoy.