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 dev kits which explain everything you need to know and provide a template file where you just need to insert your code into.