You can enlist in the USMC and apply to USNA while enlisted. There are lots of rules to this and there are no guarantees. If someone enlists with the intent to apply they should understand the risks and be prepared to serve their entire enlistment. The USNA admissions website has a section for enlisted applicants: https://www.usna.edu/Admissions/Apply/Active-Duty-Service-Applicants.php.
If appointed to USNA, you would be discharged from the USMC to attend USNA. There are also no guarantees that you will commission as a Marine after graduation if prior enlisted. Chances are high you would if you did what you are supposed to do. We had two in my class who were prior Marines who were not chosen for the USMC after graduation.
