You would want to manually edit your zone file then...
Not exactly sure about other distros, I use Ubuntu 6.06 and my zone files are located in '/var/lib/named/etc/bind
My zone file is name 'pri.giganetwireless.com
your's could as well have www
as a front appendage rather than pri
as mine does though.
Once you locate your domain zone file open it with an editor and look for the following lines to edit:
NS ns1.dnsexit.com. ; Inet Address of name server 1
NS ns2.dnsexit.com. ; Inet Address of name server 2
This is exactly how my name server line of my zone file appears, the only difference is DNSExit may have you on differently numbered name servers- I don't know, so just use what they provide you in your account.
Let me know if that helped or not?