You have the option of charging a payer whenever a payment fails due to insufficient funds. It can be any dollar amount, and it will apply to the payer's ledger when the payment fails as seen in the screenshot below. It will first return the original charge amount then with the additional NSF Fee.
If you want a specific reporting category to capture all NSF fees, that can be created in Settings > Financial > Billing Categories. The category must be an account level category, meaning you must deselect "Apply to Student Balance" within the category. Then once created, please contact our support team email@example.com to tell them which category to charge the NSF fee, and how much you want to charge.