That's fairly easy. All you need is the be29.xml file (you remove the comment 'string id' and whatever is typed after that will appear as text. For the graphics, use rugbygob.xls which is an excel file which lists all the files the game uses. They are listed into types, such as kits, minikits, logos etc. You just need to extract those filenames from data.gob (which is in the games main directory, and you use a program called 'bigGUI' to import and extract. For nearly all the team specific graphics/artwork, you need a program called 'FSHed'.
Basically, you make your kit, and graphic images in photohop, and import them into the files, and then put those patched files into the main directory, and you have a 'patch'.