Login to WHM as root
In the "Reseller Center" You'll see "Manage Reseller Main Shared/Ipless Ip" find the reseller you want and click "ok" In there you will find the option you are looking for.
It this is not function, do this:
There should be a file with the resellers username in /var/cpanel/mainips. Just edit the IP and input the resellers dedicated Ip (the one you want all his accounts to be created with).
