i extended my own bars by 2", all you need is 2 bits of old bar, something to wedge in the end (bit of wood or bar ends with a long expading wedge) and some lock on grips theres little pressure on the very end of the bars so you can get away with it, coz hands anre usually not gripping the very ends...
I did this with an old set of bars and ended up braking 2 of my grips lol. I bailed on a 360 on 2 separate occasions and the end of the bars hit the ground first and snapped the plastic body in the grip. But the bars were fine