public class User
    {
        public int UserID { get; set; }
        public string LoginName { get; set; }
        public string Password { get; set; }
    }
    public class ExternalUser : User
    {
        public int ParentAgentID { get; set; }
    }
    public class Agent : ExternalUser
    {
        public string Prefix { get; set; }
    }
    public class Client : ExternalUser
    {
        public decimal Balance { get; set; }
    }
    public class Employee : User
    {
        public int Level { get; set; }
    }

  <class name="UserA">
    <id name="UserID">
      <generator class="native"/>
    </id>
    <property name="LoginName" />
    <property name="Password" />
    <joined-subclass name="Employee">
      <key column="UserID"/>
      <property name="Level" />
    </joined-subclass>
    <joined-subclass name="ExternalUser">
      <key column="UserID"/>
      <property name="ParentAgentID" />
      <joined-subclass name="Agent">
        <key column="UserID"/>
        <property name="Prefix" />
      </joined-subclass>
      <joined-subclass name="Client">
        <key column="UserID"/>
        <property name="Balance" />
      </joined-subclass>
    </joined-subclass>
  </class>