I already answered your support ticket with the same requirement but I will copy my reply here as well if some other customers have a similar requirement.
A possible solution is to insert the additional information in the ExportOuput directly. For this purpose you need to hook OnGridExporting event handler and modify the export output. Please check out the following code snippet.
sender, Telerik.Web.UI.GridExportingArgs e)
e.ExportOutput = e.ExportOutput.Insert(0,