Setting an additional modifier key for a custom layout

Question

For anyone interested, and anyone who knows something about this, I have a question. Can I somehow use xmodmap or xkb to add a third modifier key to my keyboard layout? What I mean: If the layout allows it, you can use AltGr, Shift and the two's combination to get four characters out of each key when pressed, together with what they type on their own. Theoretically, if you were to bring in an additional modifier key (let us call it Meta, after a key from the Space Cadet keyboard), it could ramp up the number of characters per key to 8! 1: Typed Normally 2: +Shift 3: +AltGr 4: +Shift+AltGr 5: +Meta 6: +Shift+Meta 7: +AltGr+Meta 8: +Shift+AltGr+Meta Such a layout could be useful for a heavily multilingual situation, or scientific work requiring many different symbols. Sometimes, time might not allow to always go snooping around in the special character section. If anyone can tell me even just a theory on how to do something like this, I would be very glad. Looking forward to replies, and thank you in advance.