In the rare case that someone needs this, I was able to accomplish extra sites but I had to do everything through the database (not ideal). Here’s the comment I left for myself in wp_config.php:
/* If you need to add more domains to this installation it is sadly a
* ballache as there's no way of doing it through the admin.
* In the database you need to edit the following tables:
* wp_site - add a new domain row (make note of site ID)
* wp_sitemeta - duplicate all the new domain meta data
* wp_blogs - add a new domain here, match the site_id foreign key
* Then you need to duplicate the following tables:
* wp_i_commentmeta
* wp_i_comments
* wp_i_links
* wp_i_options
* wp_i_postmeta
* wp_i_posts
* wp_i_term_relationships
* wp_i_terms
* Finally go into wp_users and ensure a wp_i_capabilities and wp_i_user_level
* exists for your new site ID. You should then be able to see and edit your
* new domain.