interface IMember
{
  Account MemberAccount {get; set; }
}
public class Broker : IMember
{
  public long Id {get; set; }
  public Account MemberAccount  {get; set; }
  public string BrokerInfo {get; set; }
}
public class Agent : IMember
{
  public long Id {get; set; }
  public Account MemberAccount {get; set; }
  public string AgentInfo {get; set; }
}

CREATE TABLE [dbo].[tblAccount]
(
  AccountId TYP_ID IDENTITY PRIMARY KEY
-- login/password fields etc.
)
CREATE TABLE [dbo].[tblBroker]
(
  BrokerId TYP_ID IDENTITY PRIMARY KEY
 ,BrokerInfo VARCHAR(max)
 ,CONSTRAINT FK_Broker_Account
    FOREIGN KEY ( AccountId )
      REFERENCES [dbo].[tblAccount] ( AccountId )
)
CREATE TABLE [dbo].[tblAgent]
(
  AgentId TYP_ID IDENTITY PRIMARY KEY
 ,AgentInfo VARCHAR(max)
 ,CONSTRAINT FK_Agent_Account
    FOREIGN KEY ( AccountId )
      REFERENCES [dbo].[tblAccount] ( AccountId )
)

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
  <class name="Broker" table="tblBroker" lazy="false">
    <id name="Id" column="BrokerId" unsaved-value="0">
      <generator class="identity"/>
    </id>
    <many-to-one name="Account" column="AccountId" class="Account" not-null="true"/>
    <property name="BrokerInfo"/>
  </class>

  <class name="Agent" table="tblAgent" lazy="false">
    <id name="Id" column="AgentId" unsaved-value="0">
      <generator class="identity"/>
    </id>
    <many-to-one name="Account" column="AccountId" class="Account" not-null="true"/>
    <property name="AgentInfo"/>
  </class>
</hibernate-mapping>

  var hql = @"SELECT data FROM IMember data";
  var query = session.CreateQuery( hql );
  var members = query.List<IMember>();
  Console.WriteLine( "There are {0} members.", members.Count );