The latest version of MixRadio in Windows Phone 8.1 allows you to save a MixRadio offline Mix to an SD card.
Simply follow these steps: 1. Inset your SD card into the device. 2. Start up the Storage Sense app (available via the app list and within phone Settings) 3. Select phone 4. Select apps+games 5. Select MixRadio 6. Press the move to SD button
This will take a few minutes to finish, but once it’s done then the following will be installed on your SD card: • MixRadio app • Existing offline mixes • Future offline mixes.