Is a Redundant Ground at Sub-Panel Permitted? Useful?
What you want is a redundant grounding conductor

In your situation, provided that conduit fill allows, pulling a redundant equipment grounding conductor (bare 8AWG copper is fine) through the conduit between the panels is A-OK -- simply land it on the ground bars at both ends of the run.

The electrodes you describe would violate the NEC though

Within a structure, grounding electrodes are connected together by grounding electrode conductors, not equipment grounding conductors, though, and trying to use an EMT to do both jobs at once violates NEC 250.121:


  250.121 Use of Equipment Grounding Conductors. An
  equipment grounding conductor shall not be used as a
  grounding electrode conductor.


So, forget the ground electrodes, pull the redundant EGC, and be happy.