A way to cancel/or harness back-emf?
The back emf exists across the same terminals as the supply voltage, so it can't be separated out (I don't know why you might want to do that anyway, it isn't a free source of, otherwise wasted, energy). You can measure the back emf by driving the motor with PWM and measuring the terminal voltage during the 'off' periods - this voltage will be proportional to motor speed and can be used to produce a simple closed-loop velocity control system. Such a measurement would be via a high input impedance device (e.g. an analogue-to-digital converter) so that very little current is drawn during off-periods, otherwise the motor velocity would be compromised (any current would produce a viscous friction-type torque).