All backend side links needed should be implemented on your side.
The Metadata Adapter, once informed that a user has logged in, should communicate with your external server, get back the answers and send them to the Data Adapter, by connecting to the latter in some way.
The Data Adapter could then use this data to answer to the subscription to a user-specific item. Note that you may have to wait for the user to eventually subscribe to that item before returning the data.

If you want to simplify the process and connect to the external server from the Data Adapter, upon subscription of the user-specific item, this is possible. However, you should still ensure in your Metadata Adapter that the user-specific item can only be requested by the proper user.
Did you try this way and did you have any problem?

As we still can't understand your requirements, we can't point you to any of the available example code.