The academic writing industry is struggling right now. Writers are barely getting by as both independent and company attached writers. Going back to school to accomplish some sort of upskilling is not a bad idea. It is something that all writer's should consider at this point. Schools will not ask about your occupation as an academic writer unless it comes up as an integral part of your work experience consideration. In which case, it is never good to hide the truth. Go ahead and disclose this if the need to do so arises. The school could only look upon this in one of 2 ways:
1. You will use the school as an extension of your writing business, in which case they will refuse your admission.
2. The skills you developed over the years prove that you can complete the course you will be taking and that you are capable of working under extreme deadline pressure so they will admit you as a student.
Like they say, you will never know until you try.