Packagist is certainly convenient but not required. You can add:
"repositories": [
{
"type": "vcs",
"url": "https://github.com/ReFlar/koseki"
}
]
to your composer.json, that enables Composer to find Koseki on Github.
Then use:
composer require reflar/koseki "dev-master"
to add Koseki to your Flarum install.