Two buildings: Home and Detached Garage (circa '04 US). Service comes in through a Meter on the Garage to a PVC conduit into the Main box in the home (Single phase 240V with Neutral). In the main box Neutral and GND are bonded and bare ground wire coil heads out somewhere towards the front of the house, presumably to a ground rod.
There is an additional PVC conduit from the main panel back to the garage with 2 Hot, Neutral, and Ground that is used as two 120V runs (opposite phase), one for lights one for outlets and no sub-panel.
I'm planning on putting a 240V outlet in the garage and increasing the conductor side for 50A service. I will replace the two 120V breakers in the main panel with a 240V breaker and pull new wire (6ga x3 and 10ga Gnd) to a new sub-panel. From the sub-panel I'll wire back in the lights/outlets and the new service. Ground and Neutral will not be bonded and I know I need at least one grounding rod on this panel.
So finally I get to my question: There is an existing grounding rod that is being used by the Meter and apparently nothing else. Can I just add a new ground rod clamp to use with the sub-panel? Any code I'm violating?