Migrating VLDB to AlwaysON AG
The requirement that a database have a full backup is only enforced by the Wizard. If you just Add the database after the Availability group is already created you can restore the same backup on both the Primary and Secondary and then add the database to the Availability Group. To save time during the maintenance window we usually restore Full backups ahead of time and take a differential backup during the maintenance window.


Take Full backup of the database being moved.
Restore the full backup on the New primary WITH NORECOVERY this ensures the database isn't used until the secondary is ready.
Restore the same full backup on the new secondary WITH NORECOVERY this is needed to add them to the AG.
Restore the primary database WITH RECOVERY. RESTORE DATABASE [MyBigDB] WITH RECOVERY;
On the Primary instance navigate to AlwaysOn High Availability > Availability Groups > MyAG > Right-Click on Availability Databases > Add Database
Select your database 
Select Join only 
Connect to the secondary instance 
All the validation should be skipped 
Verify everything and click finish or Save the script. 
Verify syncronization is happening by viewing the Availability Group Dashboard