' How to: Add a new part to a package.
Public Sub AddNewPart(ByVal document As String, ByVal fileName As String)
   Dim wordDoc As WordprocessingDocument = WordprocessingDocument.Open(document, true)
   Dim mainPart As MainDocumentPart = wordDoc.MainDocumentPart
   Dim myXmlPart As CustomXmlPart = mainPart.AddNewPart(Of CustomXmlPart)()
   Dim stream As FileStream = New FileStream(fileName, FileMode.Open)
   myXmlPart.FeedData(stream)
End Sub

// How to: Add a new part to a package
public static void AddNewPart(string document, string fileName)
{
   using (WordprocessingDocument wordDoc = WordprocessingDocument.Open(document, true))
   {
      MainDocumentPart mainPart = wordDoc.MainDocumentPart;
                
      CustomXmlPart myXmlPart = mainPart.AddNewPart<CustomXmlPart>();

      using (FileStream stream = new FileStream(fileName, FileMode.Open))
      {
         myXmlPart.FeedData(stream);
      }
    }
}