Hi,
I work for a mental health children's facility. I mandate all providers, licensed and unlicesned to get an NPI number. Yes unlicensed individuals can get an NPI number. The NPI number is required on the billing forms under the rendering provider. Now since my company is a facility, they operate under OMH regulations which allow for unlicesned individuals to operate under a licensed supervising provider.
However, when researching this same scenario for my private practice ( I also have my own billing business), a lot of the private insurance companies do not allow that same scenerio unless that supervising provider was also in the room for more than half of the session. So I'd advise you to ask the insurance company what their policy is before billing in that manner.
Hope this helps.