Teh short answer is no.
SQL Statements by definition are ATOMIC.
DB2 zOS has the concept of non atomic "array insert" which may be
extended to non atomic "array merge" is some future and perhaps find
it's way into DB2 for LUW.
Until this day the only way to do it is to loop over each individual row
either in a procedure or in a the APP and perform single-row MERGE
Something like:

CREATE PROCEDURE ...
BEGIN
DECLARE CONTINUE HANDLER FOR SQLSTATE .... BEGIN END;
FOR source AS SELECT pk, c1 FROM S
DO
  MERGE INTO T
        USING (VALUES (source.pk, source.c1)) AS S(pk, c1)
        ON S.pk = T.pk
        WHEN MATCHED THEN UPDATE SET c1 = S.c1
        WHEN NOT MATCHED THEN INSERT VALUES (pk, c1);
END FOR;
END

Cheers
Serge