I'm running Kubuntu 12.04 on my PC. I want to make one USB port on my computer appear as if it is a USB mass storage device to other devices. In other words, something connected to this USB port will think my whole PC is just an external HDD or a media player or some other USB mass storage device. Is this possible? If so, how would I do it?
You really can't unless you have a special USB chip that is able to switch from being USB host to USB guest. While such chips are often present in mobile devices, on a workstation it generally doesn't make much sense (and so it isn't there).
I have a hunch that you might be asking a wrong question though.