If your midi keyboard does not have an extra usb out, you can hook the keyboard to the computer then run some software to get the usb midi in to the usb midi out of the midi bit. On windows, there is free software like VST host that does this task very well. Of course any modern DAW will be able to do this too.
If you don't want a computer involved, you will need to get a midi keyboard with regular 5 pin din midi out. The midi bit comes with a break out cable just for that. Unfortunately, lots of newer midi controllers are usb only. You will need to ask your local dealer which ones have "regular" midi outputs. It's an old standard, but it still works.
Thems the breaks. Either you use a computer as a midi manager, or you buy a new controller with oldschool midi.
However, it is possible to mod the Arduino bit to provide power over usb. It may also be possible to connect the littleBits power to the usb power outputs on the midi bit too. That would be more of a question for the support staff.