When you add new nameservers the registry performs a check to see if the nameservers you wish to add, exist on the registry. If the nameservers do exists and everything checks out you will be able to add the nameservers. However, in the event the nameservers are not correctly registered then it will yield and error and you will not be able to add them. The nameserver check is enforced by the registry and as such we cannot bypass it or override it.
In the event you are unable to add nameservers we kindly ask you please contact whomever provided the nameservers and bring this to their attention, and they should be able to correctly register them and set them up.
If you wish to add custom nameservers to your domain, you first need to create the nameservers before they can be applied. For a guide on how to create your own nameservers, please click here.