1)

You add/change your private nameserver for your dedicated server but when you create account through whm it not shows your default nameserver. You have to
add your private nameserver in wwwacct.conf
# vi /etc/wwwacct.conf
Add or edit following lines as per your requirement.
ns = ns1.domain.com
ns2 = ns2.domain.com

save the file. Now when you create account through whm it shows your servers default nameserver.

2)
If reseller client want his private nameserver add automatically when he create account through whm.

Go to whm >> Resellers >> Reseller Center

Select reseller account from dropdown list.
Click on edit privileges / Nameservers

Scroll down to the bottom of the screen and enter the name of the reseller nameserver in the blank field next to Primary nameserver and secondary nameserver
as per below.

Primary nameserver = ns1.domain.com
Secondary nameserver = ns2.domain.com

Click on save option.

Now when reseller creates account through whm it shows resellers private nameserver.