The only thing that you can do in online TurboTax is enter a Form 1099-R that reports the proposed Roth conversion and see the effect. If you do this on the TurboTax account that you use to file your actual tax return, you'll need to edit or delete this Form 1099-R to reflect the Roth conversion actually made (or not made). As xmasbaby implies, it's far easier to use the desktop version of TurboTax to do these experiments, particularly because you can create an unlimited number of tax files.
