Nameservers based on domain names are called Glued Records (e.g., ns1.xyz.org based on www.xyz.org), that can only be registered with the registrar where the domain is registered.
You may create glued nameserver records for your domains registered at iDotz.Net via the follwoing steps.
1) Log into your DMA
2) Click on the domain name you wish to create nameservers for.
3) In the Domain Services area, click 'Register/Manage Nameservers'
4) Enter the HostName prefix for your nameserver (NS1, DNS1 or ANYTHING...etc.).
5) Enter the corresponding IP numbers in the boxes provided.
6) Hit ADD to register the nameserver.
Note: If you have pre-existing nameservers that you registered on the old system, please check mark the ADD/EDIT box to modify and include them on the page.
Nameserver registration option is not available with all TLDs. Only the domains with TLDs that offer the option will have the access available to 'Register/Manage Nameservers' in the Domain Services area