If the content is hard deleted, meaning it can not be found in the database, it can not be reassigned. If posts were still in the database, but their author has been deleted, they can be reasigned to another member with a few sql queries. That is what I meant.
To answer your other question, personally I will not do such a thing (getting the deleted posts from a backup of the database, and copy their data rows into the forums_posts table and forums_topics tables).
It is not that simple imo, because more tables are involved than those 2 that you mentioned.