Follow the steps below to merge two or more of your custom playlists together -
Step 1 - Pull up your custom playlist by going to 'Your Music'
Step 2 - Select the playlist you want to merge into another playlist
Step 3 - Click edit
Step 4 - Click export
Step 5 - Save the txt formatted artist & song list to your computer
Step 6 - Select the playlist you are going to merge the txt file with
Step 7 - Click import file
Step 8 - Format - Text (Artist - Song)
Step 9 - Choose the file that you previously saved
Step 10 - Upload file