We do require that you set our name sevrers before adding the domain, as a means to verify the domain is yours to add.
Pointing a domain to our name servers IP using an A record is not going to work in any scenario, as the domain in that case would point to that server and not the one that actually hosts your files.
Technically you can change back after you have added the domain, but you'd in that case need to make sure you point the domain with an A record (the IP of your specific server), also for any subdomain you might create in cPanel you'd need to create it at the registrar and point that too.