How to merge the exported worksheet in an existing workbook?

@(Html.EJ().Grid<object>("FlatGrid")
        .Datasource((IEnumerable<object>)ViewBag.dataSource) 
        .AllowPaging()
        .ToolbarSettings(toolbar =>
        {
            toolbar.ShowToolbar(true)
            .ToolbarItems(
                items =>
                {                    
                    items.AddTool(ToolBarItems.ExcelExport);
                    items.AddTool(ToolBarItems.WordExport);
                    items.AddTool(ToolBarItems.PdfExport);
                });
        }) 
        .Mappers(map => map.ExportToExcelAction("/Home/ExportToExcel").ExportToPdfAction("/Home/ExportToPdf").ExportToWordAction("/Home/ExportToWord"))
        .Columns(col =>
        {
            col.Field("OrderID").HeaderText("Order ID").IsPrimaryKey(true).Width(90).Add();
            col.Field("CustomerID").HeaderText("Customer ID").Width(90).Add();
            col.Field("EmployeeID").HeaderText("Employee Name").ForeignKeyField("EmployeeID").ForeignKeyValue("FirstName").DataSource((IEnumerable<object>)ViewBag.data).Width(75).Add();
            col.Field("Freight").HeaderText("Freight").Width(75).Format("{0:C}").Add();
            col.Field("ShipCity").HeaderText("Ship City").Width(80).Add();
        })
)

public class HomeController : Controller
    {
        
        public ActionResult Index()
        {
            ViewBag.dataSource = OrderRepository.GetAllRecords().ToList();  
            ViewBag.data = EmployeeRepository.GetAllRecords().ToList();
            return View();
        }        
 
     }   
 

public void ExportToExcel(string GridModel)
        {
            GridProperties gridProperty = (GridProperties)Utils.DeserializeToModel(typeof(GridProperties), GridModel);            
            ExcelExport exp = new ExcelExport();
            IEnumerable data = EmployeeRepository.GetEmployees().ToList();
            IWorkbook book = exp.Export(gridProperty, (IEnumerable)data, "Export.xlsx", ExcelVersion.Excel2013, false, false, "flat-saffron", true);
 
            IWorksheet sheet = book.Worksheets[0];
 
            ExcelEngine excelEngine = new ExcelEngine();
 
            //Loads or open an existing workbook through Open method of IWorkbooks
 
            IWorkbook workbook = excelEngine.Excel.Workbooks.Open(AppDomain.CurrentDomain.BaseDirectory + "/TemplateBook.xlsx");
            IWorksheet worksheet = workbook.Worksheets[0];
 
            IRange destination = worksheet.Range["A"+(worksheet.UsedRange.LastRow+1)];//find the last row of the template workbook
            
            
            //add the exported sheet as new sheet in template workbook and copy palettes
            IWorksheet newSheet = workbook.Worksheets.AddCopy(sheet, ExcelWorksheetCopyFlags.CopyPalette | ExcelWorksheetCopyFlags.CopyAll);
 
            IRange source = newSheet.UsedRange;
 
            //copy the exported sheet data to the template worksheet
            source.CopyTo(destination, ExcelCopyRangeOptions.All);
            newSheet.Remove();//remove the added new sheet
 
            IWorksheet worksheet1 = workbook.Worksheets[0];
            IRange row = worksheet1.Range["A9"].EntireRow;
            row.CellStyle.Font.Color = ExcelKnownColors.Blue;
 
            IRange col = worksheet1.Range["B9"].EntireColumn;
            col.CellStyle.Font.Color = ExcelKnownColors.Brown;
 
            //to save as a new workbook
            workbook.SaveAs("Test.xlsx", ExcelSaveType.SaveAsXLS, System.Web.HttpContext.Current.Response, ExcelDownloadType.Open);
 
            //to save in already existing template workbook
            //workbook.Save();
            workbook.Close();
 
            excelEngine.Dispose();
            
        }