Adding existing songs to a playlist folder is simple. You've got a few options:
Drag & drop
To move a single song, simply select the song by clicking it outside of the title and drag & drop it into the playlist folder.
If you'd like to drag & drop multiple songs, select them by command+clicking them and then drag & drop them into the playlist folder.
Add to folder action
To move a single song into a playlist folder, right-click the song and select "Add to folder":
Then, select the folder that you'd like to add the song to:
To move multiple songs, select the songs by command+clicking them and click the "more" button that appears in the toolbar above:
Then, click the Add to folder button.
And select the folder that you'd like to move the songs to.