how to delegate an unmanaged zone

[#1]

My domain has a subdomain that lives on another nameserver.  I do not manage this subdomain or its nameserver, beyond delegating the name.  How can I add an NS record for a zone that is not configured on infoblox, delegated to a nameserver that infoblox knows nothing about?

[#2]

For delegations, select 'Use this set of name servers' and type in the name and IP of the name server(s) holding the domain.