Though I've only taken a cursory look at this specific library, if you're using a 3rd party library to manage users/auth, you'll want to use the library's user ID as a foreign key in your own tables rather than modifying the users table used by the library (or create a reference table if you need to have a many-to-many relationship between your table's data and users).
For the library itself, it would usually be helpful to provide some way of adding fields to the forms provided by the library as well as hooking into the user update/insert process to update/insert any data related to those fields (and a hook into the process of retrieving the data to display on the form to provide the data for the custom fields).
