whmcs multisite provisioning, add more storage

Hello, I use whmcs multisite provisioning plugin, with wp multisite (subdomain). The users can set up a subdomain under my main (tld.) domain name. I have only one top level domain name. I have one server in the server group in WHMCS, wich handles the multisite accounts. This server is assigned to my domain name and to the provisioning plugin too.
Now I would like to add new server to the server group in WHMCS, due to increase the webhosting space. And I would like to use same domain as earlier. Is it possible?
What can I do if my server become full? I would like to expand the wordpress multisite provisioning (hosting) service with more storage.

As I see if I would add a new server in whmcs which uses the whmcs provisioning plugin, I would have to use new domain name or server IP. It is not clear for me that how can I add more webhosting space / server and use whmcs multisite provisioning and use only one (my original) domain name?
Thank you for your answer.