You may be looking for this:
http://symfony.com/doc/current/book/security.html#determining-the-hashed-password
$factory = $this->get('security.encoder_factory');
$user = new Acme\UserBundle\Entity\User();
$encoder = $factory->getEncoder($user);
$password = $encoder->encodePassword('plainpassword', $user->getSalt());
$user->setPassword($password);