In the readme there is one line saying ...

2. For each client site you're producing:

a. Install the new site into the same database as the hub site, giving it a
   prefix. (Install the site as usual using the Drupal installer.)

can this work with existing sites that already have their own databases?
Or only newly created sites installed into the same (single) database that hub is already running on?

If yes, how does the hub site come to know the address of the daughter site and it's respective database?