You can, actually. I'm working on mania key layouts right now, and one thing on my todo list is to add better support for the alternate special key button. Like I said, though, it
is already supported. By default the alternate special key button is LeftCtrl. If you want it on something else, you have to edit it in manually. You need to add in an entry for Key100 in your skin.ini like this:
[Mania]
Keys: 8
SpecialStyle: 1
Key0: A
Key1: B
Key2: C
Key3: D
Key4: E
Key5: F
Key6: G
Key7: H
Key100: RightShift
Or even easier, you can change the default that I mentioned earlier by editing your osu!.Username.cfg file. The line you want to edit is this one:
keyKeySpecialAlter = LeftControl