You can also use T-SQL
use master;
go
alter availability group [ag_name] add database ReportServer
backup database ReportServer to
disk = N'\\some\shared\location\available\to\all\nodes\ReportServer_HADR.bak'
WITH FORMAT, INIT, COMPRESSION, STATS;
GO
backup log ReportServer to
disk = N'\\some\shared\location\available\to\all\nodes\ReportServer_HADR.bak';
GO
Now go to another replica and restore:
restore database ReportServer from
disk = N'\\some\shared\location\available\to\all\nodes\ReportServer_HADR.bak'
with replace, norecovery;
GO
restore log ReportServer from
disk=N'\\some\shared\location\available\to\all\nodes\ReportServer_HADR.bak'
with norecovery;
GO
alter database ReportServer set hadr availability group = [ag_name]
GO
However, what you really need to do is ensure that you're actually connected to the replica that currently holds the ReportServer* databases before you try adding things to the availability group. If you've somehow stumbled upon the secondary, or if you're connecting to the listener and the AG has failed over to what was formerly the secondary node (i.e. the replica that does not have the databases you need), then you're not going to see the ReportServer* databases.
Finally, please make sure you read the documentation.
SELECT * FROM sys.databases;
, do these databases show up? I'd venture to guess that 99% of the time when this happens, there is some confusion about which instance you're actually connecting to...