Add weight in front or behind the moving wheel? 
Consider a wheel with bearing and axle bar rolling on level ground. If I want to help the wheel continue moving longer(or faster speed), should I add weight by moving the rider body in front or behind the wheel axle bar? 
 A: Neither, however...
If you are standing on such a board, there is a simple way to propel yourself, assuming you can change direction (i.e. steer) the board.
You cannot propel yourself forward, but you can propel yourself sideways, by pushing the board to one side. This gives you some sideways velocity.
Then (before you fall over) turn the board so it is under you and moving in the direction you are moving.
Then repeat in the other direction.
In this way you can accelerate.
In fact, this is very roughly the way people propel themselves on level ground or up-hill if they are standing on anything that slides forward or backward but not sideways, like skates, skis, skateboards, you name it.
It even works in flight.
A: You can't actually propel yourself forwards or backwards in this way (unless you are taking advantage of significant friction in the bearings). Moving your body forward or backward would cause the platform to move in the opposite direction but only so much as to leave the person+platform system's center of mass unchanged. 
Another consideration. Where would the energy to propel you forward be coming from? Your gravitation potential energy is not changing and there is no mention of a power source so the situation you've proposed is one of perpetual motion, which we know not to be possible (otherwise Segways would have first appeared long ago with the invention of sufficiently smooth bearings!)
Now perhaps this isn't quite what you were asking. If your goal is to briefly speed up the rotation of the wheel in, say, the forward direction. Knowing that this isn't an actual solution you could poise yourself backwards and the wheel will spin faster in the brief moment when the platform moves forward, but again, this did not propel anything as, if you managed to stay on the board, the center of mass never once changed speed.
A real life example of all this would be people riding skateboards trying to maintain wheelies on flat ground.
