There isn't a way to do this in the UI. That said, you can likely do it with a query by selecting the thread_id and user_id from the xf_post table and inserting corresponding records into xf_thread_watch.
yup that did it. I had to filter it for uniques and run a comparison to the existing watches to weed those out. The result was anyone who has ever posted to a thread is now watching that thread. Better than no one watching any threads at all. Thanks for the tip!!!