Adding a nameserver record using BIND mass_change?

I have a number of DNS master records I'd like to add an extra slave server for. I've copied the named.conf records for my other slave server, but now I need to add a new NS record for the new slave.

It seems that it would be easiest to add using the mass_change script, but it appears to be undocumented. What I tried:

1. Selected all the master records I want to change.
2. Chose "Nameserver" in the dropdown
3. Entered my new slave in "New value" and clicked the submit button

This gives me an error because I didn't put in a current value. Can you only change existing records?