Creating nameservers (means, registering a hostname with an ip address to be a nameserver) does only work with Joker.com, if the domain used for these nameservers also is a Joker.com domain.
This is a technical/registry limitiation.
If a domain from another registrar should be used, only this other registrar is able to register these nameservers with the registry.
Of course, just using external, already existing nameservers with any Joker.com domain is fine.
To create a nameserver, you have to
- enter the Servicezone
- choose "Advanced Options"
- in "Nameserver Settings", choose "create"
- provide a hostname
- provide an ip address
- you must have the administrative rights for the domain of the nameserver