Legal Question in Criminal Law in California

It is possible to do an interstate subpoena for a criminal case, although it is a complicated process that involves the courts of both states.

I have even seen a material witness warrant from another state, where a California resident was taken into custody and transported to Utah to testify.

I wonder, though, whether you are actually being called an an expert witness. Expert witnesses are employed by attorneys in a case to provide opinion testimony about something that requires special knowledge -- ranging from scientific issues like DNA to more mundane things, like how asphalt is laid.

It would be very unusual to subpoena an expert witness, especially one who doesn't want to come to court. It sounds like you are a non-expert witness, just somebody who saw something that an attorney needs to prove a case in court.

Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.

Not if you are being called as an 'expert' witness. That can only be done with your agreement and acceptable payment to you.