' How to: Insert a CustomXMLPart part.
Public Sub XLInsertCustomXml(ByVal fileName As String, ByVal customXML As String)
   Dim xlPackage As SpreadsheetDocument = SpreadsheetDocument.Open(fileName, True)
   Using (xlPackage)
      '  Add a new custom XML part. 
      Dim workbookPart As WorkbookPart = xlPackage.WorkbookPart
      Dim xmlPart As CustomXmlPart = workbookPart.AddNewPart(Of CustomXmlPart)()
      ' Copy the XML into the new part...
      Dim outputStream As Stream = xmlPart.GetStream
      Dim ts As StreamWriter = New StreamWriter(outputStream)
      Using (ts)
         ts.Write(customXML)
      End Using
   End Using
End Sub

// How to: Insert a custom XML part.
public void XLInsertCustomXml(string fileName, string customXML)
{
    using (SpreadsheetDocument xlPackage = SpreadsheetDocument.Open(fileName, true))
    {
        //  Add a new custom XML part. 
        WorkbookPart workbookPart = xlPackage.WorkbookPart;
        CustomXmlPart xmlPart = workbookPart.AddNewPart<CustomXmlPart>();

        //  Copy the XML into the new part...
        using (Stream outputStream = xmlPart.GetStream())
        {
            using (StreamWriter ts = new StreamWriter(outputStream))
            {
                ts.Write(customXML);
            }
        }
        xlPackage.Save();
    }
}