since MPP works with PayPal, Worldpay, NoChex, Authorize.net, 2Checkout, CCBill.. It CAN be made to work with any service as long as they have a valid API Scheme. That being said I don't think its up to any Forum Software to create an API for every API out there.. I would consider this a Modification rather than a built in feature since the user of said would be in the minority rather than the majority of supported customers. I therefore would suggest contacting someone who can see how the included ones are currently created and add any that are not widely used.
Is this the official InteractivePHP, Inc. statement or your opinion?