From what I can tell, the notion of "Shift", "Alt", and "Control" are pretty well hard coded from the keyboard itself right through the OS APIs. Sometimes I see the Windows key used as a modifier, but it doesn't seem to register as a "real" modifier key, any more than any key on the keyboard would.
But could a person add a new modifier key? Maybe in a wild and insane effort to bring back the space cadet keyboard? Aside from a physically modified keyboard (or maybe just remapping some unused keys), what else would it take? New drivers? New APIs calls? A complete rewrite of the OS?
I guess this kind of thing would be a lot easier to do on an open source OS than on Windows as well, yes?
Well, there's benefit in having a standard for keyboards and mice. It makes things "work" without needing special drivers or operating system patches. But that implies that keyboards and mice have to be standardized themselves in order to have them "just work".
So, if you wanted to add an extra key to the keyboard - whether it is a modifier key or not - you have to somehow make it useful which means that applications need to be aware of it and they need a way to be notified of this key's state. Fortunately, Windows and other operating systems Open Source or not, provide an extensible and also standard mechanism for this: drivers.
So, in short you would need to provide a driver to extend the standard functionality of the standard keyboard much like you would need a driver if you bought one of those fancy mice that come with 19 special buttons and 3 scroll wheels instead of the standard two-or-three buttons and one scroll wheel.
And finally, you would need applications that are aware of your special keyboard. What use would it be to have this modded-keyboard after all if you application were ignorant of it's superpowers? The only thing you'd be able to do with your special keyboard would be to create hot-keys that incorporate your special modifier..