var ef = new ExcelFile();
ef.LoadXls("DataTable.xls");
// Find worksheet with same name as DataTable.
var ws = ef.Worksheets[dataTable.TableName];
// Append the data from DataTable to worksheet.
ws.InsertDataTable(dataTable, ws.GetUsedCellRange().Height, 0, false);
ef.SaveXls("DataTable.xls");