To create your custom grant type follow Alex's guide.
http://oauth2.thephpleague.com/authorization-server/custom-grants/
Registering your custom Grant with this package is easy:
Add the following properties to your config/oauth.php
'grant_types' => [
'custom_grant_identifier' => [
'class' => 'Your\Custom\Grant\Namespace\And\Class'
]
]