I believe the OP understands that you can only add tax liability to an existing payment plan by calling the IRS. The confusion arises from the "How would you like to pay your taxes?" question in TurboTax, since none of the available choices reflect this situation, but you're forced to choose one in order to complete e-filing.
The seemingly obvious choice of "set up a payment plan" doesn't apply here.
I propose choosing "Pay By Check" and then ignoring the check voucher as a way to complete the filing electronically but still allowing the user to adjust their payment plan manually.
