What you are describing is the issue with usb midi out only on many keyboard controllers. You will be able to power your keyboard, but the midi bit will not be able to receive the information because it is not a usb host like your computer. The only way to do this is to have a usb host receive the midi information from the keyboard and then retransmit it back to the midi bit as a separate midi out. you could build a usb to midi interpreter with an arduino, but it will require a usb host adapter shield, and then using a midi library, refire it back out a 5 pin midi port. Hope this helps.