If you mean to put an extension bar between the gopro mount on the phantom, and the gopro itself, that will work ok and lots of people do it, but.... You will have moved the center of gravity forward quite a bit and so the phantom will tend to tip forwards on takeoff. The solution... Apply a bit of back stick on the pitch control as you lift off and that will counteract it. In order to maintain level flight and hovering, the front two props will work harder than the back ones. Check for heat on the front motors on landing. It may reduce your flight times a bit as well. Adding a counterbalance weight is probably pointless as it will make the craft heavier and any advantage you got by balancing is taken away by reduced flight time.