The Pi cannot operator as a USB slave device without additional hardware. 

https://raspberrypi.stackexchange.com/questions/29621/usb-slave-emulation
https://raspberrypi.stackexchange.com/questions/271/can-i-use-raspberry-pi-as-a-usb-peripheral-device

However as hard as I try I cannot find the device I have seen previously, pretty sure it was kickstarter or indiegogo.

The Raspberry Pi Model A might be an exception 
https://www.raspberrypi.org/forums/viewtopic.php?f=98&t=74098

Good luck and lets know how you get on.