You can't add panes to playlists, but you can add playlist
groups to panes.
Drag and drop an audio view in your tree and choose copy from the menu that shows when you drop, then select the copied view and press F2 to rename it.
Now customise your copied view.
In the categories section there's an 'Add' button. Click on that, and in the resulting dialogue, you should see the playlist group option down the left hand side as shown below:
These kind of panes usually work best when used as the first pane in the view.
Tip:When you view playlist groups like this, you get 'tagging mode' check boxes when you select files in the list. You can use these check boxes to add/remove files from playlists, and if you choose the "New" option, you can easily add a new playlist to the playlist group.
Getting one stage more complex then:
This view scheme will only list the files contained within the playlist group.
Think about adding a smartlist to the group that simply lists every audio file in your library, then create a grouping rule to seperate the files that are in the playlists from everything else.
Now you can easily select other files from your library and add them to any of the playlists in your playlist pane.
Of course, there are other ways to do this, such as using the action window, or simple drag and drop, but this is quite handy because it's all self contained in a single library view.
-marko