I had this exact same issue and spent over an hour today on the phone with a Turbo Tax customer assistance rep who in turn conferenced in a "resolution" specialist who is turn was on the phone with someone else in some other office. The call ended up being cut off before anything was resolved. I was trying to persuade the reps that one of the questions should expressly address the option of a conversion of a pre-tax 457 to a post-tax 457.
Meanwhile, on my own, I decided that absent some change to the software in the next 2 weeks, I'll resolve the issue the way that the first person who responded to your question proposed.
