To add your basis info from a 2016 Form 8606 that was not prepared by TurboTax, after entering the Form 1099-R for the Roth conversion, click the Continue button on the Your 1099-R Entries page, answer Yes when asked if you made and tracked nondeductible contributions to your traditional IRAs, then enter your total basis as of December 31, 2016 that was shown on line 14 of your 2016 Form 8606.
