However, I would like to access a foreign ticker in another database, preferably using AFL. This ticker is actually a composite ticker. Can this be done?
You can only access symbols in the current database.
So you should move that data you want to access to current database. It is just a file. You can copy it with Windows explorer to appropriate folder http://www.amibroker.com/guide/h_workspace.html The only thing to watch for is that if you move data files you should delete broker.master file so it gets recreated next time you load database.