I have the "PROLIFIC usb to serial converter" (blue, 5 pin usb to serial converter). it's working to program my arduino and everything already. what I want to do, is turn one of my atmega328s into a USB keyboard peripheral. can this be done? I know that you can use the arduino board as a keyboard/joystick if you reprogram the converter on it, but can something similar be done with an external TTL device?
It would be dumb, since any board with a 32U4 processor can do that, with full existing software support. In fact, I doubt that what you are asking about is even possible.