Is there a way to move multiple individual posts that are in one thread, into another existing thread? To be clear, I don't want to merge two threads, just certain posts that are in one thread into another existing thread?
Select both the posts.
Go to the bottom of thread
Inline thread moderation
Split posts.
Edit : Didn't read another existing thread, sorry.
Edit:
Still possible, split those posts into new thread, then merge the splitted thread with existing.
Why is not this plugin core I don't understand
I agree, it should be in core.
Were's that list of features for 1.8 to add this in
Already posted there but will be a long wait for 2.0. I think 1.8 should see this plugin as core.