in odoo partner menu you can add account bank detail regarding that user what i mean is if you want to record that partner/customer account bank you should add that field in the ecommerce/website in so the user can add their account bank detail. but if you want to record the paypal or credit card data you should add new column or model then add that column in the website so you can record it becoz there is no feature to record credit card or paypal account in the backend there is just column to add bank account number regarding that user

Great, that's exactly what I want to do. However, I don't know how to modify and add fields to the checkout form and automatically populate the bank details of the partner record. Do you know of a tutorial or can you point me to some doco that might help me?

https://www.odoo.com/documentation/8.0/