RadControls – RadGrid export to Excel with text format

        void btnExport_Click(object sender, ImageClickEventArgs e)
        {
            rgGrid.ExcelExportCellFormatting += new OnExcelExportCellFormattingEventHandler(rgKBuser_ExcelExportCellFormatting);
            rgGrid.MasterTableView.ExportToExcel();

        }
        void rgKBuser_ExcelExportCellFormatting(object sender, ExcelExportCellFormattingEventArgs e)
        {
            e.Cell.Style["mso-number-format"] = @"\@";
        }

Reference: http://www.telerik.com/help/aspnet-ajax/grid-html-export.html