Fluent mapping inheritance not by SubClass?

Question

Hello, I have problem with mapping my classes by fluent nHibernate. My database looks look like this I have two tables Product and DocumentPossition. DocumentPossition knows about Product but product doesn't know about DP. Now on the database DocumentPossition has foreign key which is connected to Id of Product. DocumentPossition has it's own Id.

On C# Product is a parent of DocumentPossition.

    public class Product
    {
      public int Id {get; set;}
      public string Name {get; set;}
    }

    public class DocumentPossition : Product
    {
      public int Id {get; set;}
      public decimal Net {get; set;}
      public decimal Gros {get; set;}
      public int Amount {get; set;}
    }

I have no idea how to map this. I can not use SubclassMap because this can not have second Id maybe somebody knows how to make this by Fluent nHibernate.