How to change role hierarchy storage in Symfony2?

In my project I need to store role hierarchy in database and create new roles dynamically. In Symfony2 role hierarchy is stored in security.yml by default. What have I found:

There is a service security.role_hierarchy (Symfony\Component\Security\Core\Role\RoleHierarchy); This service receives a roles array in constructor:

public function __construct(array $hierarchy)
{
    $this->hierarchy = $hierarchy;

    $this->buildRoleMap();
}

and the $hierarchy property is private.

This argument comes in constructor from \Symfony\Bundle\SecurityBundle\DependencyInjection\SecurityExtension::createRoleHierarchy() which uses roles from config, as I understood:

$container->setParameter('security.role_hierarchy.roles', $config['role_hierarchy']);

It seems me that the best way is to compile an array of roles from database and set it as an argument for the service. But I haven't yet understood how to do it.

The second way I see is to define my own RoleHierarchy class inherited from the base one. But since in the base RoleHierarchy class the $hierarchy property is defined as private, than I would have to redefine all the functions from the base RoleHierarchy class. But I don't think it is a good OOP and Symfony way...