How to add a dynamic list member to a domain component (DC) at runtime?

I need to separate related dc´s into different modules so I can sell extension modules which upgrade existing domain components.

Example: My base module contains the dc "ICustomer" which has Invoices

C#
[DomainComponent]  
  public interface ICustomer  
  {  
      int CustomerNumber { get; set; }  
      string Name { get; set; }  
      IList<IInvoice> Invoices { get; }  
  }  

  [DomainComponent]  
  public interface IInvoice  
  {  
      int InvoiceNumber { get; set; }  
      ICustomer Customer { get; set; }  
  }  

What I want to do is to separate the financial functionality into a separate module that I can add on customer request.
So I need to define the base module like this:

C#
[DomainComponent]  
  public interface ICustomer  
  {  
      int CustomerNumber { get; set; }  
      string Name { get; set; }  
  }  

And the FinancialModule like this:

C#
[DomainComponent]  
public interface IInvoice  
{  
    int InvoiceNumber { get; set; }  
}  

Now this two modules are independent. What I need now is the aggregation of customer->invoice.
I tried to implement this similar to How do I define a custom member for a domain component (DC) at runtime?: (see my comment there)

C#
public WinSolutionWindowsFormsApplication()  
       {  
           InitializeComponent();  
           this.SettingUp += WinSolutionWindowsFormsApplication_SettingUp;  
       }  

       void WinSolutionWindowsFormsApplication_SettingUp(object sender, SetupEventArgs e)  
       {  
           string customMemberName = "Invoices";  
           Type customMemberType = typeof(IList<IInvoice>);  
           TypeInfo domainComponentTypeInfo = (TypeInfo)XafTypesInfo.Instance.FindTypeInfo(typeof(ICustomer));  
           IMemberInfo memberInfo = domainComponentTypeInfo.FindMember(customMemberName);  
           if (memberInfo == null)  
           {  
               memberInfo = domainComponentTypeInfo.CreateMember(customMemberName, customMemberType, "");  
           }  

           customMemberName = "Customer";  
           customMemberType = typeof(ICustomer);  
           domainComponentTypeInfo = (TypeInfo)XafTypesInfo.Instance.FindTypeInfo(typeof(IInvoice));  
           memberInfo = domainComponentTypeInfo.FindMember(customMemberName);  
           if (memberInfo == null)  
           {  
               memberInfo = domainComponentTypeInfo.CreateMember(customMemberName, customMemberType, "");  
           }  

       }  

But the creation of the aggregated list does not work.

What´s the correct way to create an aggregation for dc´s at runtime?