dynamic route

hi, how can i add new routes dynamically? directly in the controller, like this:

PHP Code:

class Users extends CI_Controller {

    public function __remap()
    {
        $route['username'] = 'users/profile/user_id'; // username and user_id r from the db
    }

    public function profile($user_id)
    {
        // ...
    }
} 


the __remap also can be in MY_Controller, it doesn't matter, the main is the result

any idea?