You can use this tool to merge two or more Rdio playlists together.
To start using Playlist Merge, click/touch the banner at the bottom of the window. A popup will appear asking for permission to access your Rdio account. Obviously we need to be able to copy your current playlists and push the new ones to your account.
Merges will result in a new playlist, leaving the original playlists as they were. To delete your old playlists, open them in Rdio and use the "edit" button at their top.
NOTE: This tool is offered as is, for free, and with no guarantees. It worked when I used it for myself, but it's not like I unit tested the thing.