Mecanim - human rig, only one bone for the foot?

Hi

To have a better and more realistic control of the foot (especially big ones) a Mecanim human rig should support more than one only bone for that extremity. How other can one animate a rolling of the feet?

Is there a way to integrate any additional bones in a rig to give it to the control of Mecanim? In my case I also have a comic character which has a standard human rig, but also a tial and a trunk.