Is it possible to add user roles beyond those defined by default by Directories Pro?

This currently can only be achieved through custom code. The below would allow for adding custom roles:

add_filter('drts_wordpress_roles', function($roles) {
    $roles[] = 'my_user_role';
    $roles[] = 'my_other_user_role';
    return $roles;
});

In a future release, addition of a settings interface for enabling/disabling custom roles is planned. (I assume this also means that one will be able to add/remove custom roles using this same settings interface).