If the Excel file is just missing the extension "xls" or "xlsx" you can change the extension using C#:
* If is not excel, then you need to create an excel file, fill it with your data, according to the specific parser of your data etc.
If you just want to copy the file to another location and change it's name, use CopyTo:
string DestinationPath = @"D:\NewFile.xls";
FileInfo fi = new FileInfo(@"C:\SoureFile.xls");