Nexus 6 dev prev3 here. Its been done, on stock, for years with any file manager out there - you select mp3 and tap set as ringtone. It would be set as one, and the file would be permanently listed amont all other system provided sounds from the rollout menu. Now the functionality is gone. set as ringtone doesnt change it and doesnt add it to the list, which forces us to use stock ringtone, notification and alarm tone. Did anyone found a workaround to this?