In the .big file, you have a file called kit. You need to extract this, use fshed to import a 512x512, 1024x1024 or 2048x2048 edited kit template image into it, and save. Then open the .big file again and import with the name it is in the file, which is 'kit'.
Simples.
btw that London Welsh jersey is very easy. You just need a 'tight' kit template. Add a nike logo to the left, london welsh to the right and google basildon chemicals logo and cut out the background. Basic photoshop. Change the shirt to red, and match that colour on the kitback. You can see where the shorts are. There is a step on the right side, where socks meet shorts. Black all over. the other part is the sleeves. Colour this red, same hue as the red before. save. Done.