If you wish to add a key to the list, you must remove both the checksums (bytes 4 & 8), and all spaces between bytes. So your example becomes 12F9A13CDEDE. Then, using a Windows text editor, you can add or insert that line. If adding at the end, be sure to press <RETURN> to include the line break.
You might also consider creating your own "knowns.txt" with new keys, and search that one before the shishmish file.
Using a Windows editor ensures that the line-breaks are CR-LF (two characters, so in total 14 bytes per record in the file).