I want do delete a forum category but first, move all the posts in that forum category to another. Can this be done or do I have to move each thread seperately via the "Thread: Move" option?

I don't think there's a bulk move, so you'll have to write something to do it for you. If you fetch all posts that have a forum_id_fk of the forum you've deleting and also have a post_root_id of 0, then those will be all the threads in the forum. Then you can use GForum::Post::move to move the post.

Adrian