Inheritance mapping with concrete base class

Hi,
in my domain, i have the following entities:

public class User {

public virtual int Id { get; set; }
public virtual string Name { get; set; }

}

public class Employee : User {

public virtual int EmployeeNo { get; set; }

}

both Users and employees are stored in the same table. The EmployeeNo column is a nullable int.
How can i configure FluentNHibernate mappings to create an instance of User when the EmployeeNo column is null, and an instance of Employee when EmployeeNo is not null?

Using standard .hbm.xml mapping files, i achieved that purpose using the following configuration:

<id name="Id">
    <type name="Assigned" />
</id>
<discriminator formula="case when EmployeeNo is not null then 'E' else 'U' end" type="String" />
<property name="Name" column="Name" />
<subclass name="Employee" discriminator-value="E">
    <property name="EmployeeNo" />
</subclass>

Thank you
Riccardo