SonataUserBundle/Resources/config/doctrine/BaseUser.orm.xml

<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
                  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                  xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                  http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

    <mapped-superclass name="Sonata\UserBundle\Entity\BaseUser">

    </mapped-superclass>

</doctrine-mapping>