Actually, I do not agree with the statement "Usually you don't gain much by using such a sensor". Although GPS is great for average speed and overall distance, GPS is terrible for current speed. Modern foot pods are really accurate with respect to current speed and cadence. I would really like to see speed taken from those instead.
No, running / walking cadence sensors are not supported. Usually you don't gain much by using such a sensor because:
* Speed is not very accurate - it's more accurate through GPS
* Acceleration sensors are already present in the phone, so you can detect step frequency through those (which is already done by Run.GPS)