It is possible to host a subdomain here and not the main domain, but you have to change the nameservers of the main domain in order to do that.
I would just push a message to your users saying that maintenance will occur starting at x time, then switch your nameservers, add the subdomain here, then switch them back.
Ok. I switched the nameservers, created the subdomains and then I switched back. I see the subdomains listed in my account and can create additional subdomains if I want What do I do next?
Ok. I used the IP address from the client area and updated A records for both subdomains on the DNS on godaddy where I host the main site. I installed Wordpress on both subdomains through softaculous on IF. Both seemed to install correctly but I am not able to access the admin URL. I can obviously give it some time, but wondering if there might be something wrong.