In this post Andy Beaulieu describes adding Behaviors at runtime for the Physics Helper Behaviors.
These behaviors are a little different in that they are dependent on a PhysicsControllerBehavior as kind of a “master controller.” There are also dependencies for some of the other Behaviors – for example a PhysicsJointBehavior depends on the existence of a PhysicsObjectBehavior for the bodies it is joining. So there is an additional call necessary for some of the Physics Behaviors (this may be fixed in a future release).