public void Extend( 
   NetworkDiagramExtendType extendType
)

public void EditDiagram(NetworkDiagram diagram, List<Guid> globalIDs)
{
  //     These routines generate their own editing transaction, and therefore cannot be wrapped
  //     in a separate transaction. Because the editing performed by these routines cannot
  //     be undone, thise routines can also not be called within an editing session. All
  //     edits in the current edit session must be saved or discarded before calling these
  //     routines.

  // refresh the diagram - synchronizes it based on the latest network topology
  diagram.Update();

  // append features to the diagram
  diagram.Append(globalIDs);

  // overite the diagram with a set of features 
  diagram.Overwrite(globalIDs);

  NetworkDiagramInfo info = diagram.GetDiagramInfo();
  if (info.CanExtend)
  {
    diagram.Extend(NetworkDiagramExtendType.ExtendByContainment);

    // or extend for only a set of utility network globalIDs
    diagram.Extend(NetworkDiagramExtendType.ExtendByContainment, globalIDs);
  }
  // delete a diagran
  diagram.Delete();
}