Those addons can be used by Agile Toolkit.

1. Install Agile Toolkit into "atk4"
2. Install addons into "atk4-addons"

3. Inside your API include addons through addLocation():

   $this->addLocation('atk4-addons',array( 'php'=>array('mvc', 'billing/lib', 'misc/lib', ), 'template'=>'misc/templates/', ))->setParent($this->pathfinder->base_location);

Only include search path for addons you will need. Addons will not be initialized unless you specifically use them.