There currently isn't a way to merge a complete thread with another and I never wrote anything to select an existing thread to merge even some (or all) posts in another thread with it. 

Thanks for your prompt reply.

I'm looking to port my current sites away from postnuke/pnphpbb2 and over to dotnetnuke.  Because we have high volumes of traffic, we often see duplicate threads started and often each instance has something worthy to keep.  So we typically merge them into the primary starter thread.

Is this a feature you are considering and if so, how soon do you think it would be implemented?

Thanks so much!  (and also thanks for the search tip!)