As jack9 proposed higher up in this thread, you could put the EmDrive on the outside of a rotating system to provide constant torque, which accelerates the rotation. Then wait until the system has sped up to the point where the kinetic energy increases faster than the power you put in. Then use the whole thing to drive a generator.
And anyway, kinetic energy isn't a conserved quantity. It can be converted into potential energy in a gravity well, or lost entirely in inelastic collisions. Momentum is the quantity that is always conserved, and that is what the emdrive violates directly (though still not in a way that allows for perpetual motion or free power, near as I can tell).