You are better off creating a new payment profile for the payment processor rather than modify an unused one. XenForo supports an unlimited number of payment profiles, so there's no reason to not have a new one. You can of course start with the code from an existing one to help you along.
All you need to do is tell XenForo that there is a new payment profile available by adding a record to the xf_payment_provider table. In there, you tell XenForo what the class is for your new payment provider(s). As an example, I have 2 additional payment providers (one for Bitcoin and one for Monero). My xf_payment_provider table looks like this:
...anyway, long story short is you can add your own payment provider, you don't need to modify an existing/unused one.
Teaching you how to code an addon is really outside the scope of this thread. Ask around to n the developer forum and I’m sure you can find some help. I was strictly just saying that you don’t need to override/modify existing (but unused) payment providers because XenForo doesn’t have a limit on how many you can have.