yes this is possible.
Use the auth-pam plugin to authenticate users; then use a 'client-connect' script to map the authenticated username to a specific IP. The 'client-connect' script can write out client-specific include options using
script-security 2
client-connect </full/path/to/script/file>
I'd use a simple text file (or mysql database) to match the usernames against the IPs you want, e.g.
user1 ip1
user2 ip2
user3 ip3
The script would do something like
#!/bin/bash
IP=`awk "/$username/{ print $2}" client-ips.txt`
echo "ifconfig-push $IP 255.255.255.0" > $1
