How to enable motors in Arduino Robot's motor board when powered with USB

Greetings everybody, I am new to the forum and the Arduino community.

I try to control the Arduino robot (http://arduino.cc/en/Main/Robot#.UxJM6PldW5A) with a Rapberry Pi via USB serial connection. The software put together works perfectly but the motors won't move because: "For safety purposes, the motors are disabled when the robot is powered from the USB connection." is there a way to override this limitation?

Thanks in advance!