Adding a database to existing distributed availability group
I am working in a test situation and have already successfully created a distributed availability group with automatic seeding.


Automatic seeding is a property of the replica not a distributed availability group.


  I cannot figure out how to add another database to the distributed AG.


Interestingly I went to find the MSDN BOL page to show this... and there isn't one. That definitely needs to be corrected and I'll follow up on that.


  I successfully added the new database to the primary AG, but the alter availability group statement does not seem to work on the distributed AG.


Distributed AGs don't hold databases, they hold availability groups. Availability Groups hold databases, that's where we need to go.


  Surely there is a way to add a database after you create the distributed AG?


Surely there must be, as to create a distributed availability group the secondary availability groups must be void of all databases.

How to add a database to an existing distributed availability group:


Create the new database on the distributed availability group's primary replica for that availability group.
If you are using automatic seeding, it should seed the database to the other replicas in the primary replicas group. If you're not using automatic seeding, backup and restore the database to the secondary replicas leaving it in a restoring state. Then join the database to the local availability group.
Regardless of the automatic seeding setting, you'll need to backup and restore the database to each secondary distributed availability group. Restore the database and leave it in a restoring state. Then join the database to the local availability group. Do this for each secondary availability groups' replicas.
Have yourself a treat because that's a bunch of work.