Different nameservers for subdomain

Hello!
I want to clear something out. Let's say that we have the followin address site.com and the nameservers are ns1.site.com ( ip: 1.2.3.4 ) and ns2.site.com ( ip: 4.5.6.7) .
If i want a subdomain like sub.site.com to point to another nameserver ( ip: 7.8.9.1) so that i can independently create subdomain like 2018.sub.site.com and add A/MX/Cname records directly from this server ( 7.8.9.1) then what records should i add to the first 2 nameservers?
something like this?
ns1.sub.site.com in NS 7.8.9.1

I hope i explained as clear as possible
Thank you!