There different settings related to users:

User Model

user.model = Kwf_User_Model

Change this setting to use a custom user model. Your own user model has at least to extend Kwf_User_Model.

User Forms

user.form.self = Kwf_User_Form
user.form.grid = Kwf_User_Form

With these settings you can change the form used when editing users. Using that you can add new fields to a user.

Pasword Validator

user.passwordValidator = Kwf_Validate_Password3of4

Using that setting you can change the validator used on passwords. Set to false to have no validator at all.

The default is fairly strict (and relatively secure)