Answer
No — it isn't possible to switch to a different window directly from within a PL/SQL Block-type Custom Menu's own executed code; that specific action type can only run PL/SQL logic, not perform a client-side window navigation. The practical, if not maximally user-friendly, solution is to keep two separate Custom Menu items: one of type PL/SQL Block for the actual logic/data change, and a second of type Create-Window-With-Transfer for the actual window navigation, executed manually one after the other by the user, since there's no way to combine both actions into a single PL/SQL-Block-type Custom Menu.