User {
   id: Integer,
   firstname: String,
   middlename: String,
   lastname: String
}

tbl_Users {
   user_id: Integer,
   user_firstname: String,
   user_middlename: String,
   user_lastname: String,

   primary key(user_id) auto_increment,
}

Administrator extends User {
   login: String,
   password: String
}

tbl_Administrators {
   administrator_id: Integer,
   administrator_login: String,
   administrator_password: String,

   primary key(administrator_id),
   foreign key(administrator_id) references tbl_Users(user_id)
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="ru.sql.domain.User" table="tbl_users">
        <id name="id" column="user_id">
            <generator class="identity"/>
        </id>

        <property name="firstname" column="user_firstname" type="string" not-null="true"
            insert="true"
            update="true"/>

        <property name="middlename" column="user_middlename" type="string" not-null="true"
            insert="true"
            update="true"/>

        <property name="lastname" column="user_lastname" type="string" not-null="true"
            insert="true"
            update="true"/>

        <joined-subclass name="ru.sql.domain.Administrator" extends="ru.sql.domain.User" table="tbl_administrators">
            <key column="administrator_id"/> <!-- 1 вариант -->
            <!--<key column="user_id"/>-->    <!-- 2 вариант -->

            <property name="login" column="administrator_login" type="string" not-null="true"
                insert="true"
                update="true"/>

            <property name="password" column="administrator_password" type="string" not-null="true"
                insert="true"
                update="true"/>
        </joined-subclass>
    </class>
</hibernate-mapping>