EAP-PEAP with "special" users

I've inherited a manky old RADIUS server and I'm trying to get to grips with it. In short, it is using EAP-PEAP/MSCHAPv2 to authenticate users against Active Directory. This is all working fine. I want to add some test user accounts that can be authenticated through EAP but use a flat file rather than ntlm_auth. What's the recommended way of handling this without harming the performance of the majority of the users who will not appear in the users file? Thanks, Jonathan