can a CRNA volunteer as RN?

You are a RN right! you are working under your RN license. when I was a ER tech, a couple of RNs and I volunteered at a triathalon and they worked as RN as volunteer. I live in california. As far as I know the only time you cant work as a medical professional is if you are on like a ride-a-long in an ambulance or medi-vac.

From what you say CRNA, you are a RN with the anesthesia education/specialty

I don't see why not. You still maintain your RN license with the state you are practicing in.

I don't see why not, I'm fairly certain that you have to keep your state RN license active as a CRNA. I'm curious, what exactly would you want to be volunteering (I hope this doesn't sound bad but why not just get paid, like perdiem?)

Heed TraumaRus' message. You can practice (for pay/volunteer) in a capacity as an RN or even an LPN position BUT you will be held to the standards of care of an advanced practice nurse. Just as an ER/trauma nurse is held to a higher standard of care when they help out at an accident scene and they need to have insurance because the Good Samaritan Laws don't always cover nurses that have specific knowledge.