Yes! Open the Playlist Panel and then click the Import button. Choose which provider you want to import from and follow the prompts.
For YouTube imports you will need to type in a YouTube username and then a list of that users public playlists will appear. Click on an item in the list to see its contents. Click the back button to choose a different playlist.
For SoundCloud you will have to first authenticate plug.dj to access your SoundCloud playlists. Once you grant access to your SoundCloud account you can import your Favorites, Sets or Tracks you have uploaded yourself.
To add items to your playlists just choose whichever item(s) you want from the resulting list and then drag them to the playlist you want.