Don’t add more sites to point more domains to the same site. Just add more domains to the same site from this address /wp-admin/tools.php?page=domainmapping of the dashboard of the site.
You might need to disable primary domain check from this network admin page /wp-admin/network/settings.php?page=dm_admin_page if you don’t want the domains to all redirect to the primary one.
For support about the domain mapping plugin, you should go here: https://wordpress.org/support/plugin/wordpress-mu-domain-mapping