TurboTax doesn't really have a "native" interview or process for stock sold with NUA. You might need to figure out your results outside of TurboTax and then make entries in TurboTax that come to that result. If you had sold that stock on the same day it was distributed to you you would have reported LTCG in the amount of the NUA. Price movement since the distribution date is considered short term gain or loss or long term gain or loss depending on how long you owned the stock before selling.
You say you "have the cost basis at that time" (of distribution I assume) and that's because a 1099-R was issued to you since it was a distribution from a retirement plan. That cost basis is still your cost basis.
Since the stock was moved to an after-tax brokerage account its sale should result in you eventually receiving a 1099-B which you might have to break up into two sales if you have short term gain or loss resulting from price movement from the time of distribution to the date of sale.
Tom Young
