I don't think there is a super fast way.
You could do it like this:
* in "POSTS" tab -> select needed posts -> save those posts to .txt file (you can choose to save to single file) -> then edit that file and add <new-subject> tag for each post (basically just copy paste first line and surround with <new-subject>SUBJECT</new-subject>
* Then use "Replace posts with new posts from .txt":
http://manager.themasoftware.com/faq.php?expand=faq403Just load that .txt file it will match them by subject and when editing will use the new subject, there is an option to search in all websites for the same posts.