public class OrganizationMap : IAutoMappingOverride<Organization>
   {
      public void Override(AutoMap<Organization> mapping)
      {
      }
   }

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" default-access="">
  <class name="Orders.Core.Organization, Orders.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" table="Organizations" xmlns="urn:nhibernate-mapping-2.2" />
</hibernate-mapping>

public class UserMap : IAutoMappingOverride<User>
   {
      public void Override(AutoMap<User> mapping)
      {
         mapping.JoinedSubClass<Organization>("ColumnId", m => {...}
         );
      }
   }

public class MyAlteration : IAutoMappingAlteration
{
    public void Alter(AutoPersistenceModel model)
    {
        model.ForTypesThatDeriveFrom<User>(
            map => map.HasMany<User>( x => x.Children)
        );
    }
}

model.Alteration( a => a.Add<MyAlteration>());

mapping.JoinedSubClass<Organization>("UserId", m =>
    {
       m.HasMany(x => x.Currencies).Element("Currency").AsBag();
    }
 );

public class OrganizationMap : IAutoMappingOverride<Organization>
   {
      public void Override(AutoMapping<Organization> mapping)
      {
         mapping.HasMany(x => x.Currencies).Element("Currency").AsBag();
      }
   }