puttygen

Create keys as follows:
fbsdbox>ssh-keygen -t rsa -b 2048 -f xxxx_notices_key -o -C xxxx_NOTICES

-t rsa                    type rsa
-b 2048                    key length 2048
-f xxxx_notices_key    filename xxxx_notices_key
-o                        save private keys using new OpenSSH format instead of pem
-C xxxx_NOTICES        comment

Then you will need to change the password using ssh-keygen to convert it to a format puttygen will use, then use puttygen to convert it for pageant.

fbsdbox>ssh-keygen -p -f extcmpny_company_key
Enter old passphrase:
Key has comment 'extcmpny-company-key'
Enter new passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved with the new passphrase.

fbsdbox>puttygen extcmpny_company_key -o extcmpny_company_key_for_putty.ppk
Enter passphrase to load key:
fbsdbox>ls
extcmpny_company_key                extcmpny_company_key_for_putty.ppk
extcmpny_company_key.pub


To show the fingerprint and some other information:

fbsdbox>ssh-keygen -l -f xxxx_notices_key.pub
2048 SHA256:+Gy/PLS/g5IpvyDteK8oxpyNI8h4bf5fXsyi3itkdII xxxx_NOTICES (RSA)