As per your last comment, you can import the data from a datatable (dataset) to a new worksheet in an existing workbook using the following code sample:
//Open the existing workbook
Workbook workbook = new Workbook("book1.xls");
//add new worksheet other than the existing worksheets
Worksheet worksheet = workbook.Worksheets.Add("New Sheet");
//import data from datatable (dataset) to newly created worksheet
//dataTable is the dataset table
worksheet.Cells.ImportDataTable(dataTable, true, "A1");
//save the updated workbook
workbook.Save("book1.xls");