No, there is no extension. Typically, if you set the value of your 'Bank Transfer' option to -1, and that value is used for the payment gateway order value setting, all that will happen is the pending order will sit there waiting for you to activate it in the back end. nBill will not send an e-mail (other than the pending order confirmation), but you could add some PHP code to send an e-mail out either in the form submit code setting, or using the pending_order_created event.