Answer this question with "Pay By Check", and finish filing. Turbotax will instruct you to download a form 1040-V payment voucher afterwards - this is what you would use to mail a check. Then you can call the IRS customer service line, and ask to have your debt rolled into your existing plan. If they agree, then you can ignore the 1040-V and don't need to send a check.
You will, of course, have to pay for TurboTax up front using a credit card.
