How to add multiple rows to a table that contains only a generated always identity column?
One way is to use MERGE (best thing since sliced bread?). 

MERGE INTO T x
USING (
    SELECT 1 FROM SRCTABLE
) y (dummy)
    ON 1=0 -- Always false
WHEN NOT MATCHED THEN
    INSERT (x) VALUES (DEFAULT);


This will add one row to T for every row in SRCTABLE.

It would be cool if we could grab the generated values with the use of transition tables, but it appears as if this does not work for MERGE (db2 10.5, have not checked 11 yet):

SELECT * FROM NEW TABLE (
    MERGE INTO T x
    USING (
        SELECT 1 FROM SRCTABLE
    ) y (dummy)
        ON 1=0 -- Always false
    WHEN NOT MATCHED THEN
        INSERT (x) VALUES (DEFAULT)
); 

SQL0104N  An unexpected token "select * from new table (" was found 
following "BEGIN-OF-STATEMENT".  Expected tokens may include:  "&lt;space&gt;".  
SQLSTATE=42601


Compare with:

SELECT * FROM NEW TABLE (
    INSERT INTO T (x)
    VALUES (DEFAULT)
);