What is Reseller Center? This set of features allow you to add or remove reseller status on existing accounts, email resellers, apply ACLs to resellers, reset package permissions, and configure existing reseller accounts.
Go to WHM and click Reseller Center under Resellers
At Reseller Modifications, Choose or highlight reseller username to be set. This set of features allows you to modify the way your resellers interact with your web server.
Now try to Select edit/Privileges and nameservers
Scroll to bottom of the page for the Nameserver entry and add the private name servers.
Also make sure that A records are setup for the private name servers in the zone file as well.