How can I achieve this?

wp-config.php does not have host information.
Installing Redis Object Cache does not give me any host information (it’s deactivated and I can’t change that).

The “Access to website” drop-down only allows us to select a single site. However, once we’ve added the new user with [email protected] address, we can click on a menu icon to the right of the user entry, and use the “Add Websites” item to add sites (one at a time, repeating the action until we’ve added all the sites we want you to migrate).