The usb port has four connections, +5Volts at up to 500mA, GROUND and 2 high speed low voltage differential signal lines. You could not do this with a direct connection to the usb port.
The only way to do it would be to use a Micro with an inbuilt usb interface programed to output the pulses. Microchip have pic chips that could do this.
Check out microchip usb examples.