How to use PuTTY keys with the ssh-agent

If PuTTY is installed on Windows™, start PutTTYgen, load your private key (ppk file), and select the conversions >> Export OpenSSH key menu item. It will ask you where you want to save the file.

If PuTTY is installed on Linux™, execute puttygen input_key.ppk -O private -o output_key.