Juan All you need is the PPK, The passphrase to the PPK and puttygen.exe to generate the Public Key and export the private key in an openssh format. You could copy the public key from the ~/.ssh/authorized_keys file but this file can contain multiple public keys making it harder to validate which one is correct.
Start by loading the existing PPK file into puttygen.exe. You will be prompted for the passphrase. Once completed you can see the public key that was generated from that privake key.
Next create test file called Public.key and paste the contents you see in the key section created with the generate.
Then go to the Conversions menu and export openSSH key to a file called Private.key
Now when select SSH-2 Public Key Authentication in the PAM credential manager you can load the Public.key, Private.key and the same passphrase and start using this credential.