If you want send the DNS domain name to the client over PPP dialup, it is not supported.
But you could pass down the ip address of the DNS server. There are 2 ways to do it-
1) global config:
async-bootp dns-server
2) in latest 12.1 and 12.2 IOS versions, you can configure it under the interface terminating the
async PPP users (typically a group-async interface):
conf t
int group-async 1
ppp ipcp dns
Configuring WINS, DNS and DHCP on Access Servers-
http://www.cisco.com/warp/public/793/access_dial/winsdhcp.html