You can create own user entity which will not extend SonataUserBundle User.php class.
If you used FOSUSerBundle:
class User extends FOS\UserBundle\Entity\User
or you can extend by default Symfony 2 Security User.php class.
Next you can configure SonataUserBundle to use your user entity:
sonata_user:
class:
user: MyBundle\Entity\User
Full configuration is here:
https://github.com/sonata-project/SonataUserBundle/blob/master/Resources/doc/reference/advanced_configuration.rst
If you used SonataUserBundle default controller to manage users you need to create own UserAdmin Class and configure sonata:
sonata_user:
admin: # Admin Classes
user:
class: MyBundle\Admin\UserAdmin