How to convert .iso to .xiso? OG XBox

As the topic suggests. What program does vimms lair use to create their xiso files from iso files? Specifically for the OG XBox. There are some iso files that can't be downloaded from vimms for various reasons and I have found larger iso files of those games that l'd like to convert to xiso with the same settings/ quality as the xiso's found on VimmsLair. Please help.