Well, it's not as easy as your proposed change. A CollisionNode is not a GeomNode and does not contain any Geoms. I believe the way collision geometry is stored is different than the way normal geometry is stored.
This is more a feature request. But really, do you *want* to make it accept collision geometry besides ordinary geometry? If so, I'd really like to know the reason.
Also, collision geometry is really only meant for Panda's collision system and I'm not sure if all ODE users want it, so we'll have to be careful not to break existing code that depends on this behavior.