Hi Devang,
As far as I know, ip name server is only for local use on the router. It will not translate your user queries.
As well, PIX will not support this. You need to specify to the users the real ISP DNS and to permit the traffic.
The alternative is to put a DNS server in your LAN/DMZ that will use the ISP DNS as proxy.
Please rate if this helped.
Regards,
Daniel