Change nexus 8 brake I bought a Shimano Nexus 8 IGH with shimano nexave, but I would like change it to disc brake. 

Is it possible install a rotor disc brake adaptor? Will it work?
When you are braking you are putting quite some load on the frame where the brakes are installed. (Newton's 3rd principle of dynamics: to each action corresponds an equal reaction with same intensity and opposite direction)

If the frame is not designed for this, you are aiming for disaster. 

So, if the fork was not designed to have disc brakes installed, don't do it.