I do not know such a free tool,but you can try below code:
<pre lang="c#"> Workbook workbook = new Workbook();
workbook.LoadFromFile(@"..\excel1.xls", ExcelVersion.Version97to2003);
Workbook workbook2 = new Workbook();
workbook2.LoadFromFile(@"..\excel2.xls", ExcelVersion.Version97to2003);
Worksheet sheet2 = workbook2.Worksheets[0];
DataTable dataTable = sheet2.ExportDataTable();
Worksheet sheetAdd = workbook.CreateEmptySheet("Country");
sheetAdd.InsertDataTable(dataTable, true, 1, 1);</pre>