Hello Elias,
This is possible - you can change the Login logic for any application to suit your needs. However, it would be considerably easier to use the existing OutSystems Users logic/data model - there would be less development time and maintenance required.
Can you tell us a bit more about your usecase? You could consider automating User creation and having a Timer that periodically fetches new Users/Permissions from your database connection, and then writing them into the User Entity. This way, you could maintain the existing authentication logic.