Only way I know of is to get the doctor's report that confirms spread and have your lawyer ask them to start paying for it. If they don't then take them to court with the report from your doctor and ask them to order work comp to start paying for it. They should because if not for your work accident you would not have RSD and therefore it would never have spread. So it's all related and should be covered.

My experience has shown me that the most likely scenario is having to take them to court about it. They may even want to send you to an IME (though if they just sent you to one recently then I don't know if they will be able to) to see if their doctor agrees with your doctor's assessment.

Good luck. Definitely start the process as soon as possible just in case it takes a while to get those approvals.