Unfortunately bank accounts and credit card accounts are the only options that NS gives you. Ideally, you're looking for it to be an expense account? It would seem that it would at least need to be an asset or liability account. But regardless, you'll have to create it as a bank account. Unless you go fancy and bypass the bill payment transaction altogether and make use of a custom transaction to pay off the bill. Then you can use any account you want. But that may be overkill and not fit in with your overall business process.