How to add ssh key to gpg-agent in Windows 10?

[Luis Marsano]

I'm using GnuPG 2.1.11 from the *Simple installer for GnuPG modern* Windows
binary release on https://www.gnupg.org/download/index.html
After muddling about a great deal, I found gpg-agent can be enabled as an
ssh-agent for PuTTY by

   - adding enable-putty-support to *gpg-agent.conf*
   - gpg --expert --edit-key key-id
   and adding a subkey with authentication flag
   - gpg --list-secret-keys --with-keygrip --with-colons key-id | sed -ne
   '/:a:/,/^grp/ {/^grp/p;}' | cut -d: -f10 >>~/.gnupg/sshcontrol
   to get keygrip and add it to *sshcontrol*
   - gpg --export-ssh-key key-id | ssh user at host 'cat -
   >>.ssh/authorized_keys'
   - reloading gpg-agent

Is there a simpler way to add the keygrip to *sshcontrol*?
The above enables us to use gpg-created keys for ssh sessions.
The manual
https://www.gnupg.org/documentation/manuals/gnupg/Agent-Options.html#index-enable_002dputty_002dsupport-62
also discusses using *ssh-add* to add ssh keys to gpg-agent.
PuTTY, however, doesn't include *ssh-add* and cygwin/msys2 *ssh-add* doesn't
seem compatible with native gpg-agent.
How do we add ssh keys to gpg-agent in Windows?
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