Creating Modules
From WHMCS Documentation
If you want to integrate WHMCS with a payment gateway that isn't supported, or automate account creation on a propriety control panel, or register domains with a domain registrar that isn't supported, then you will need to create your own module.
With WHMCS, we make this a very easy process by providing documentation and templates for each type of module which make it as easy to implement a module as the API you are integrating.