Currently it is not possible to insert custom worksheets (for example notes or a cover page) when exporting via ExportToExcelML
The SpreadExport (http://www.telerik.com/help/winforms/gridview-exporting-data-spread-export.html) functionality of RadGridView features the WorkBookCreated event, which allows to introduce any final customizations to the exported SpreadSheet. Since this export method uses the SpreadProcessing API, it should be used to modify the workbook (http://www.telerik.com/help/winforms/spreadprocessing-overview.html).