No you wouldn't be guaranteed an appointment. Many non-service academy prep schools, not NAPS or West Point's Prep School etc., say that have a 90+% appointment rating. There is some chance you might not get appointed. You would have to get a nomination again, you cannot get an appointment without one. There are programs that all service academies use to send prospects to prep schools other than their own prep school. Usually, however, you cannot apply for it, you are more or less chosen for it, similar to the fact that you cannot directly apply for any of the service academies' prep schools. You may want to read exactly what those type of schools actually offer, from NMMI's website. Also, Valley Forge Military is another one that has a service academy prep program.