Actually, this is not provided by PLSD. Applications that use windows like PLSD does (parent/child windows) will have those shortcuts available from the Operating System. If you press Alt-Space from any child window, you'll see the standard menu, which includes the "Next" option (and its shortcut). I'm not sure it is a good idea to change standard Windows behaviour (this comes from Windows 1.0).
Regards,
Gustavo