NOTE: The OP requires Command line. This answer uses a graphical application... Will leave the answer here just in case it is useful.

There is a possibly simpler solution.

1. Download (if you do not already have it) the PuTTY Key Generator (PuTTYgen) application by going here.

2. Open PuTTYgen

3. Click Conversions, then click import key

4. Locate your key file, then click open.

You can now save your key as a PPK file by clicking the Save private key button

You can also save a public version of your key by clicking the Save public key button

https://devops.profitbricks.com/tutorials/use-ssh-keys-with-putty-on-windows/

https://kb.site5.com/shell-access-ssh/how-to-convert-ssh-keys-to-ppk-format/

NOTE: The OP requires Command line. This answer uses a graphical application... Will leave the answer here just in case it is useful.

There is a possibly simpler solution.

1. Download (if you do not already have it) the PuTTY Key Generator (PuTTYgen) application by going here or here.

2. Open PuTTYgen

3. Click Conversions, then click import key

4. Locate your key file, then click open.

You can now save your key as a PPK file by clicking the Save private key button

You can also save a public version of your key by clicking the Save public key button

https://devops.profitbricks.com/tutorials/use-ssh-keys-with-putty-on-windows/

https://kb.site5.com/shell-access-ssh/how-to-convert-ssh-keys-to-ppk-format/

There is a possibly simpler solution.

1. Download (if you do not already have it) the PuTTY Key Generator (PuTTYgen) application by going here.

2. Open PuTTYgen

3. Click Conversions, then click import key

4. Locate your key file, then click open.

You can now save your key as a PPK file by clicking the Save private key button

You can also save a public version of your key by clicking the Save public key button

https://devops.profitbricks.com/tutorials/use-ssh-keys-with-putty-on-windows/

https://kb.site5.com/shell-access-ssh/how-to-convert-ssh-keys-to-ppk-format/

NOTE: The OP requires Command line. This answer uses a graphical application... Will leave the answer here just in case it is useful.

There is a possibly simpler solution.

1. Download (if you do not already have it) the PuTTY Key Generator (PuTTYgen) application by going here.

2. Open PuTTYgen

3. Click Conversions, then click import key

4. Locate your key file, then click open.

You can now save your key as a PPK file by clicking the Save private key button

You can also save a public version of your key by clicking the Save public key button

https://devops.profitbricks.com/tutorials/use-ssh-keys-with-putty-on-windows/

https://kb.site5.com/shell-access-ssh/how-to-convert-ssh-keys-to-ppk-format/