Since you said the rudder only goes full right and full left, there's little chance that the radio in the plane is advanced enough to do the modfication that you want to do. It would take moving the rudder servo (the motor that makes the rudder go left-right). Adding ailerons to the wing then making the rudder servo move the ailerons. Which as you said will now only go full throw. Not a very easy situation to control. You'd be far better off buying a better "simple" foam and electric trainer like this cub:


http://www.hobbyzone.com/rc-planes-p...cub-bl-rtf.htm