I can't bind Ctrl+Shift+<another key>? Or Alt+Shift+<another key>?

I play multiple games, using a n52te keypad for the left hand and a Logitech G5 for the right hand. The Logitech is configured so that the four additional buttons are Ctrl+Shift, Alt+Shift, Ctrl, and Alt, allowing me to use those buttons in combination with the keypad to drum up a lot of options.

I use this combination in three other games, without issue (except for the nagging Ctrl+Shift+0 issue in WoW...), but I somehow can't use it here because three-key combinations aren't accepted as a valid key binding.

Is anyone else having this difficulty, and is this a known issue?

Known limitation with the input handler.
In Champions Online we were hoping to define 'Virtual Keys' or variables linked to a UI state (like a certain window being shown etc.) so we make interface specific controls.

Right now you can only have 2 keys in a bind <key1>+<key2>.

Basic Key Binds and Command List
might prove useful. Not sure if you can define more than 1 binding per line (haven't seen that or tried it myself).

Oh forgot to mention this thread:
Xbox 360 Controller Keybinds
and I do recall that you can combine inputs from different devices so you may just want to reference them as joybuttions.