class Program
    {
        static void Main(string[] args)
        {
            string strDoc = @"..\..\Documents\Sheet9.xlsx";
            Stream stream = File.Open(strDoc, FileMode.Open);
            OpenAndAddToSpreadsheetStream(stream);
            stream.Close();
        }
        public static void OpenAndAddToSpreadsheetStream(Stream stream)
        {
            // Open a SpreadsheetDocument based on a stream.
            SpreadsheetDocument spreadsheetDocument =SpreadsheetDocument.Open(stream, true);

            // Add a new worksheet.
            WorksheetPart newWorksheetPart = spreadsheetDocument.WorkbookPart.AddNewPart();
            newWorksheetPart.Worksheet = new Worksheet(new SheetData());
            newWorksheetPart.Worksheet.Save();

            Sheets sheets = spreadsheetDocument.WorkbookPart.Workbook.GetFirstChild();
            string relationshipId = spreadsheetDocument.WorkbookPart.GetIdOfPart(newWorksheetPart);

            // Get a unique ID for the new worksheet.
            uint sheetId = 1;
            if (sheets.Elements().Count() > 0)
            {
                sheetId = sheets.Elements().Select(s => s.SheetId.Value).Max() + 1;
            }

            // Give the new worksheet a name.
            string sheetName = "Sheet" + sheetId;

            // Append the new worksheet and associate it with the workbook.
            Sheet sheet = new Sheet() { Id = relationshipId, SheetId = sheetId, Name = sheetName };
            sheets.Append(sheet);
            spreadsheetDocument.WorkbookPart.Workbook.Save();

            // Close the document handle.
            spreadsheetDocument.Close();

            // Caller must close the stream.
        }
    }

class Program
    {
        static void Main(string[] args)
        {
            string strDoc = @"..\..\Documents\Sheet9.xlsx";
            Stream stream = File.Open(strDoc, FileMode.Open);
            OpenAndAddToSpreadsheetStream(stream);
            stream.Close();
        }
        public static void OpenAndAddToSpreadsheetStream(Stream stream)
        {
            //Initialize a new Workboook object
            Workbook workbook = new Workbook();

            //Load workbook from stream
            workbook.LoadFromStream(stream);

            //Append a new sheet to worksheet and then name it
            int count = workbook.Worksheets.Count + 1;
            workbook.Worksheets.Create("Sheet" + count);

            //Save workbook to disk
            workbook.Save();
        }
    }