Put in a new "first post" in a thread?

[Tonje!]

I have some posts in a thread which I want to move and make a new thread out of. But I don't want the oldest post moved from the old thread to be the first post in the new thread - if I'm making myself clear now :p I want to make the first post of the new thread myself, and then let the posts from the old thread follow.

How can I do this?