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