How do you add an external directory to repository?

I want to add an external directory to an existing repository.

External Dir: /home/some/directory

Working Dir: /htdocs/.git

If I attempt the following command from the /htdocs dir:

git add /home/some/directory

I get an error: fatal: '/home/some/directory' is outside repository

I understand that I can add a symbolic link to /home/some/directory, but I believe that only track the link. How do I track all files under the directory as well?

Thanks in advance!