Hi Fabian,
I haven't worked with EXECUTE IMMEDIATE, but if your procedure has an output table, then you could do the following:
-- 'outer' procedure
CREATE PROCEDURE TEST_PROC AS
BEGIN
-- assuming you have a procedure with output table called SOME_OTHER_PROC
CALL SOME_OTHER_PROC(a);
-- a was populated in procedure above, has same structure as output of that proc.
-- you can treat it as a table
SELECT * FROM :a;
END;