Reply Attribute and Stripping a realm

I have a couple questions. I need to force a reply attribute for the slipstream service to all my customers. I'm using flatfile, just a basic setup. What would be the best way to do this? Also, How do I strip Realms? We get users coming to our RADIUS in this format username@realm.com My flatfile only has username due to the backend system we use. I need to strip the realm.. Thanks