This is Mikey with Airship Technical Support. Just to clarify, are you attempting to set an Alias or a Named User? Named User has superseded Alias as a way of identifying multiple devices for one user. Have you had a chance to look at our documentation on How To Implement Named Users? This would be a good first step to learning how to associate named users with channel ids.
Happy to assist further if needed!
Airship | Technical Support Engineer | Portland, OR