Help: How to authenticate additional attribute

Hi, I am running FreeRadius to authenticate with a WinXP SP2 client using PEAP on WPA2. However, in addition to the normal username and password authentication, I would like to authenticate the client with its location coordinate i.e. the authentication will be based on username, password and location coordinate. I understand that WinXP provides a dialog box that allows me to input the username and password. Can I use the domain field to input the location coordinate? If this is possible, what must be configured or done to the FreeRadius so that it may authenticate this additional attribute? Do I need to modify the source code? Is there anybody who may advice me on this? Thanks. With Regards, Chew Heng Hui Andy