To add additional items to a playlist
that you have created...
... From an individual item page, you can click on the Select Playlist
dropdown, choose the existing playlist and click "Add to Playlist"
or, if you are on the Music list page, you can choose multiple dropdowns to
the right of each single you want to add and then choose "Add to Selected
Playlists"
Upon returning to your "My Playlist" area, you will see the newly added
items in the playlist you selected.