Supplying power to a KSR10 Robotic Arm with USB

The KSR10 Robotic Arm with the optional USB kit still requires 4 huge batteries to operate.
This is offcourse rather strange, since the arm is now connected to a USB port, which has the ability to supply +5v.
The arm motors only use 3v.

The standard setup uses the 4 batteries in a -3v - 0 - +3v setup, so there is no simple way to just connect the +5v usb power to power the robot’s motors.

Does anyone have a suggestion for a easy setup to prevent me from changing D-cells every time?

(The easiest thing i can think of right now is putting 4 NiMh aa-cells in the compartiment to provide the 2x3v, and integrate a small 220v or solar charger. This still requires one extra cable, and i prefer to use only one cable: USB. But the solar thing is still a nice idea)

USB port is not capable of supplying required current.

+1, USB ports are rated to supply a maximum of 500mA, and many cannot supply even that. I have seen devices that use dual USB connections to draw power from two ports, however often these will also fail as the hub is not capable of proving the full 500mA to two ports simultaneously.