It's possible and that is all I can tell you because there is a multitude of things to consider when making that.
You need to be able to refer back to the database which tells you if a username or email has already been used. And so on.
My advice. Just enable uNet and you'll be set
The difference between local and uNet is you will not be able to delete users from your site because uNet accounts are globally recognized across uCoz. Local you can.