Unfortunately the Subselect method does not support joins. Hope you can solve it some other way!
Also note that the usage of 'AS' in select statements is not supported by the SQL connector.
You can write:
SQL SELECT EKKO~KNUMV
INNER JOIN EKPO ON EKKO~EBELN = EKPO~EBELN
WHERE EKPO~AEDAT BETWEEN '20080601'
AND '20160630' AND EKPO~LOEKZ <> 'L';