If you connect your phone to your computer, then access the SD card there should be a folder called 'media' in it. Open that and create a folder called 'Audio', then inside there make folders with the name Alarms, Notifications or Ringtones - then put whatever audio you want to correspond to the type in the appropriate folder.
Seems pretty unintuitive eh, but it works. I just added all the turret noises from Portal to my notifications, it's gold
