Alternate means of grounding/bonding panel

[AFGHAN Spark]

I am working in Afghanistan. I have a 400a panel being fed by two 500kcmil multi-conductor cables. The included egc combined size is to small. These conductors run underground and come up into the panel in PVC. I have an MDP upstream, which is where the Neutral/Ground are bonded. How can I safely ground this panel? The tent is supplied with two sub-panels, each panel has the same problem, and are fed from the same source. Can I bond the building steel and run a conductor to a rod?

 

[jumper]

Using multi-conductor cables for a parallel installation often violates 250.122(F) because of the undersized EGC.

The addition of a GES, ground rod, will do nothing towards alleviating this problem.