Can you be more specific about what you're trying to do?
If you want to modify nameservers for a domain registered through us, you need to do this from the Domains -> Registration section of the panel (click on the domain name).
If you want us to delegate a subdomain to other nameservers by adding NS records (and potentially glue records) for you, you'd need to contact support.
You don't want to leave the nameservers pointing to us with the registrar, but have different nameservers listed as the NS records for the domain with our nameserves. Trust me.
If I haven't explained what you're trying to do, perhaps you could explain in more detail.