How To Pointing NS elsewhere in Cloudflare?

I have a domain name calvyn.com, transfer into cloudflare as my domain registrar.
NS pointing to
aida.ns.cloudflare.com
dane.ns.cloudflare.com

I am using a service name builderall. With using their drag and drop website builder, I can simply point my subdomain A record to their IP eg esp.calvyn.com or I can simply point my primary domain A record to them to build my website there. (no issue for this part)

Wherest they also have WordPress builder, and when I would like to point my domain to there, either primary domain / sub domain, A record is not working. (here the issue)

According to builderall support, the domain name require to point to their NS in order to connect to their WordPress builder.
eg. ns3.l4dns.com and ns4.l4dns.com

from cloudflare, I found no way for me to change the NS to others.

from such scenario, is there any workaround?

There are nameserver options on the paid plans, but no work around on the free plan.