You are allowed to alter ANY OF THE ASSETS YOU WANT, DIRECTLY! Literally, anything you can edit, you are welcome to edit. When you cook a Total Conversion, you need to create an empty sub directory in the Mods folder for the Total Conversion to cook properly.
These are the most powerful form of new content, and they are listed in the 'Total Conversions' section of the main menu, because each Total Conversion is effectively a new game.
