There is no built-in method to sync an already existing event. You have to create it that way. However, you COULD add a sync via the database.
Table: xxx_dtregister_group_event (where "xxx_" is the prefix of your database)
The second field is "eventId". For a non-synced event, this will be "0". Simply enter the ID number of your JEvents event here in place of the 0 and the sync will be added.