Actually, when you use the "Tools & Settings" > "Database" menu, you can access all databases through phpMyAdmin at once as you are logged in as admin. Simply click on the phpMyAdmin/maintenance icon at the right edge of the list of database servers (where normally only one or two will be shown).
And if you want to access all databases at once, you can simply add a suitable user to the "users" table of your database, allowing access to all databases. Build it similar to the admin user, but don't try to call it admin or root.