Qwer
I still doubt that freezers are not cold enough to kill a large percentage of bacterias. Bacterias frozen in labs will be frozen in something different than water. It probably totally depend of the species...
But if they die, I don't think they will regrow. That would be like adding a bacterial culture to pasteurized sauerkraut to make it a probiotic food again. That's unlike getting unfermented meat out of the freezer.
Anyway, the bacterias themselves might be much less important than the substances they've created in the natto.
Domo