public class Union
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
}
public class UnionMap : ClassMap<Union>
{
    public UnionMap()
    {
        Schema("dbo");
        Table("Union");

        Id(p => p.Id).Column("Id");
        Map(p => p.Name).Column("Name");
    }
}

public class Plan : Union
{
    public virtual int Shifr { get; set; }
}
public class PlanMap : SubclassMap<Plan>
{
    public PlanMap()
    {
        Schema("dbo");
        Table("Plan");

        Map(p => p.Shifr).Column("Shifr");
    }
}

...AddMap(m => m.FluentMappings.AddFromAssemblyOf<Union>());
...AddMap(m => m.FluentMappings.AddFromAssemblyOf<Plan>());