Can you make an unmarked police car if in law enforcement?

My friend just got into law enforcement and he has a car that he always drives, and loves. Apparently, he is able to put in some dash ons for the controls such as the siren control box, and the lights control box. He just put in the siren and everything but is working on finishing his lights. Are you allowed to turn your own car into an unmarked police car IF you are in fact in the law enforcement?