Sean,
Absolutely.
If you look in /App_Code/payments.vb there are two code sections that handle the current providers: one is called AuthorizeNet, the other PaypalPaymentsPro. You would add a new region, probably by copying AuthorizeNet (I imagine it will be closer to that API than to PayPal's).
Change the method name to ProcessPaySimple() and implement your custom PaySimple logic in there.
Search for the function called ProcessPayment() in the same file, and add ProcessPaySimple() to the case statement.
Add PaySimple to the dni_processors table, and add its required authentication tokens (username, password, etc...) to dni_processors_config. The new processor will magically appear in the processor configuration section and you can set the values there.
That should just about do it. Feel free to email us directly at support@dotnetinvoice.com if you run into trouble while making this change.
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Rob
DotNetInvoice Support