The payments options are actually implemented as plugins -- you'll see them in plugins/paymethods. (Even the manual option.) A number of folks have created (or have discussed creating) additional plugins, mostly, similar to the paypal plugin, and which are available on the forum. You should be able to either a) extend the current manual payment plugin to allow registration, say upon providing a key (a date, etc.); or b) use the paypal plugin as a starting point to create a plugin specific to your banking method, if such a thing is possible.
If you have any other questions, please let us know.