no we do not have a rider to add that coverage, it's in our standard policy. I had to check because we used to do a very large event that utilized up to a dozen volunteers in order to keep costs down (and involve more of their people). Our liability reads that accidents to or caused by volunteers are inclusive. Employees injured would be covered by workers comp, but damages they might cause would be covered by our liability. The only weakness in insurance we really have is damage to our own equipment...there are a couple instances where simply we are on our own for cost of repair/replacement.
(edited to add the above is an abbreviation and oversimplification of our coverage...it's not verbatim from our policy and terminology used is probably a little lax on my part)