How to extend Ion_auth

Hello

Which is the correct way to extend a third_party library / model ?

In particular, I need to modify some existing functions in original library / model, or add new ones.

But without modify the originals PHP files...

Example: how to extend Ion_auth libary or Ion_auth_model...

Thanks

I renamed the original model to Ion_auth_b_model.php
After that, I created a new model named Ion_auth_model.php
This model starts with the following code:

PHP Code:

require_once APPPATH . 'models/Ion_auth_b_model.php';
class Ion_auth_model extends Ion_auth_b_model
{
    public function __construct()
    {
        parent::__construct();
    }

   //After here my extended functions  
}