Add new fields to signup form

I'm working on a custom authentication plugin (based on email-based self-registration with admin confirmation - https://moodle.org/plugins/view.php?plugin=auth_emailadmin) and I need to add some fields to the signup form. I can't use profile fields because one of the new fields will be a list of courses from Moodle, and will be dynamic. Is there any way I could to this from the plugin, without modifying Moodle source code?