modifier for denial

Is there an appropriate modifier that can be used to bill a PPO insurance to get a denial so I can bill Medi-Cal. We provide CPSP services to our Medi-Cal patients and some of them have private insurance as primary. These PPO insurances refuse to send me a denial, they are continually sending me letters requesting a new HCFA with valid CPT® codes. Any help would be appreciated!

Thank you!