How to use a custom sysprep file with Create Disk Image task

I see reference to people using custom sysprep answer files with GSS 3.x, but how is this accomplished? I don't see anything on this topic in the guides. I tried to just check the 'Prepare using Sysprep' option, but the default options it sets aren't appropriate. Is it possible to import a custom sysprep answer file and process it using the 'Prepare using Sysprep' option? Also, any documentation on what settings are in the default answer file?