One way would be a new screen listing categories and forums and the user simply navigates down through them to the post they want to merge with. Simple but not exactly elegant.

It might be more useful to have a new screen with a selectable list of subject lines from the forum you are already in, listed below a copy of the post you have selected to merge. It could work somewhat like the post_reply_write feature in the way it displays the original post text below the reply form. Then you could include a category/forum pulldown and have it repopulate subject lines if a new forum were selected giving you move and merge.

Suspect some other folks will have some good ideas on the subject, but that's what comes to mind from the top of my head.