Appending Excel files

as 'bsb25' suggested below one approach would be to use excel interop from your C# code, however another approach would be to use an excel's library for C#.
For example something like the following:

ExcelFile requestBook = ExcelFile.Load("Translation Request.xls");
ExcelFile dictionaryBook = ExcelFile.Load("Translation Dictionary.xls");

// Copy second row from "Sheet1" in "Translation Request.xls" file
// into a "Sheet1" in "Translation Dictionary.xls" file
// onto an "A2" position (the beginning of the second row).
requestBook.Worksheets["Sheet1"].Rows["2"].Cells.CopyTo(
dictionaryBook.Worksheets["Sheet1"], "A2");

dictionaryBook.Save("Translation Dictionary.xls");

You can also refer to this article about reading excel files from C# application.