Normally a git repository is not treated as a special remote, but as a git remote of the normal kind. Two exceptions to that are git-lfs and gcrypt special remotes.

But it is possible to register a git repository as a special remote. git-annex and git will use the remote the same as any normal git remote, but its url will be recorded in the repisitory. One benefit of doing this is it allows git-annex init to autoenable the remote.

First you need a regular git remote with the url that you want to use for the special remote.

git remote add tmpremote ssh://...

Then, to set up the special remote:

git annex initremote myremote type=git location=ssh://... autoenable=true

The location must be the same url as the existing git remote.

Now git annex init in each clone of the repository will autoenable myremote.

Note that the name of the git remote (tmpremote above) has to be different than the name you later use for the special remote, since git-annex initremote will refuse to use the name of an existing remote. To work around that, you could finish by removing tmpremote and enable the special remote:

git remote remove tmpremote
git annex enableremote myremote