NTLM Authentication and send NAS-IP-Address param.

Hello, I'm using poptop to authenticate in freeradius. Freeradius is autenticating on a Active Directory using NTLM and it is working fine. (MS-CHAPv2 + MPPE 128bits ) But, I would like to fix ip address by username in my vpn connection. I know that "NAS-IP-Address" is the way, but where can I set this parameter? All my vpn users are located on Active Directory (Windows 2003 Std) Is it possible to send NAS-IP-Address parameter when I'm using NTML authenticantion? And... How can I do it? :) Thanks in advanced! Domingo Rodriguez