Setting up automatic credit card processing

[glowgirl]

For people who choose not to use PayPal and Google Checkout, I process authorized credit cards on file manually through my account at ProPay.com.

 

I realize that WHMCS can be set up for automatic gateways (like authorize.net), but I'm wondering if there is a way that I can set up a payment method apart from the ones included in WHMCS?

 

e.g., can I create an option on checkout for people to have their cards billed automatically by completing my authorization form?

 

Also, if I am manually charging a credit card upon invoicing, can I designate that certain clients get a credit card invoice rather than a billing invoice?

 

Thanks.