Yes, please submit a request to the Flex Help Desk and let us know the company or state domain that needs to be added. (Personal email domains are not allowed.) Once the extra domain is added you can add and archive (remove) users as needed. Be sure to periodically review which individually added users still need access as they will not be archived through regular syncing (file or API).
For more information about adding new users individually, please reference the "Flex Admin Guide” which can be found by clicking on the HELP > Documentation > Flex Training Document Index within the Flex system, and click the eyeball.