Moderator’s don’t have the ability to split posts into a new message the same as admins, but they can convert a whole topic into a PM using the option in the topic wrench.
Sorry, I may have misread what the issue was.
If the private area is another category rather than a PM, then Moin and Jagster are correct. You’d change the category of the topic rather than use the ‘select post’ functions. You can only create a new topic when splitting out posts rather than moving a whole topic.