It certainly is possible--by adding another web application to your system. Here's roughly how I have done it:
Your primary web app uses Forms authentication. On the forms login page, any user that is determined to be on the local LAN (check IP address), redirect them to another app that uses Windows authentication. In this second app, you can determine the user (assuming the browser is configured to send credentials automatically to the zone in which your app resides), then set a cookie which your first app can read, and redirect the user back to the original app.
This does work.