This how to will show how to convert id_rsa keys that were already created on Linux, without a passphrase, to .ppk extension so it can be used with Putty on a Windows box.
Download PuttyGen from here and open it. Once it opens click on Conversions => Import Key
Search for the id_rsa key on you computer
Click on “Save Private Key” and “Yes” to save without a passphrase.
Choose a location and a name for the new .ppk key