Surely, one can assign several character codes to the same keyboard key and invoke them by pressing 1, 2, 3, 5 level modifiers together with this key. No cycling through all the possible characters by pressing an extra special key though.
A good introduction to the Linux keyboard can be found here
and here
askubuntu.com What does "Key to choose 5th level" in gnome-keyboard-properties do? gnome, keyboard, xmodmap asked by sup on 09:41AM - 08 May 11 UTC