Kathleen Dollard has a very good article on  that talks about using Managed Extensibility Framework (MEF) to  allow customers to add their own forms to our Windows Presentation Foundation (WPF) app?  

In this simple example she show adding custom defined forms into the standard menus.. all of this is done in VB!

Redmond Developer: Working with MEF

<Export(GetType(IExtension))> _
<ExportMetadata("MenuCaption", "First")> _
Partial Public Class First
   Implements IExtension

   Public Sub ShowWindow() _
      Implements Common.IExtension.ShowWindow
   End Sub
End Class

Enjoy!