Welcome to Openfire. As you noticed “services” provided by Openfire are implemented as subdomains of the main xmpp domain. That includes external components as well as internal components. Moreover, Openfire does not have support for virtual domains.
External components need only to specify the “subdomain” they are going to host. That information is passed in the initial stream header and the server will append the “main xmpp domain” to compose the complete address of the component. The new service should be accessible for local and also remote users.
For remote users to find the new service it is recommended to add DNS SRV records for the new service. However, if remote users are using another Openfire server then you may skip adding a DNS SRV record for that service since Openfire will first try to resolve subdomain.domain and if that fails it will try to resolve domain and if that worked it will connect to that address.