Basically the opposite MultiSite URL question that most people ask
One way to achieve this, is to alter the way WordPress maps the request to a blog in your multisite network by using the sunrise.php drop-in. Sunrise overwrite the default setup in wp-includes/ms-settings.php. There, a bunch of global variables are set up like $current_blog or $current_site. To activate this dropin you have to define the … Read more