You can enable database for stretch using T-SQL and to existing Azure Database

ALTER DATABASE <database name>
 SET REMOTE_DATA_ARCHIVE = ON (SERVER = server name);
GO

Check if Database is enabled for Stretch

use master
go
select is_remote_data_archive_enabled,name from sys.databases where is_remote_data_archive_enabled=1
go

example
ALTER DATABASE [SQL2016DB] SET REMOTE_DATA_ARCHIVE = ON (SERVER = N'remotedataarchive-lenovo-pc-sql201-sql2016db-20151011-172130496.database.windows.net')