Why should you do that? If an IP is visible as Free it's not use by users or the hosting. So ip's for secondary nameservers are normally "free".
As you can see from the link you pointe yourself to, there are also a lot of "free" ip's.
They should be added in the ip manager, but as long as they are not used for a user as dedicated ip or used for the server as shared ip, it's in fact "free" to be used for something else. Otherwise you would never have free ip's.
If it's used by the secondary nameserver it also says free, for the same reason, you can safely ignore that, it's working like designed.