More recent versions of Portage allows you to add additional repositories by
adding a config file in /etc/portage/repos.conf
. Here's an example:
[3PO-overlay]
location = /var/db/repos/3PO-overlay
sync-type = git
sync-uri = git://github.com/3PO/3PO-overlay.git
auto-sync = yes
Keyword as appropriate and use emerge like you normally would do.
Contributions are welcome. Fork and create a pull request.