Limbs are all about rotation, so that is the only parameter available for the Curve Editor to work with.
Changes in position that you accomplish with the gizmo have to be translated in to rotation settings for the various bones using IK.
Adding position in 3D space would make things very complicated, because what would be the reference?
Lastly, the Curve Editor plugin is owned by RL, so making changes (which would require reverse engineering) is against the EULA.