The issue is that the DB is online on a secondary node of the AG, and you want to add it to the AG. This can be done quite easily, albeit with a small amount of downtime. This method is based on the steps to Manually Prepare a Secondary Database for an Availability Group.
Step 1) Take a full backup of your database, and restore it
WITH NORECOVERY to all other AG nodes. (You can also use an existing recent full backup.)
Step 2) Take a transaction log backup of your database, and restore it
WITH NORECOVERY to all other AG nodes. (If you used a recent full backup in step 1, you'll need to apply all transaction logs since then to bring the restored DB current.)
Until this point, steps are identical to manually preparing the AG secondary.
Step 3) Take a tail log backup of your database. This is done by performing
BACKUP LOG....WITH NORECOVERY. This will take your database offline and put it into "Restoring..." status.
Step 4) Restore the tail log backup to only the AG primary node, using
WITH RECOVERY, which will bring the database online & accessible on that node.
Step 5) Proceed with the steps to join a database to the AG. This can be done either via the SSMS GUI or T-SQL or PowerShell.