Can you "upgrade" an existing user to have admin privileges?
I see that users are stored in a users.pickle file in ...sage_nb/ But the API doesn't seem to have a way to change the admin property. Only to get or create them.
This question: http://ask.sagemath.org/question/497/changing-the-name-of-a-notebook-user asks the similar one of changing the name. It appears you have to programmatically delete and re-create the user.
Creating a user is done with add_user() documented here.