How to modify the files in *.appx file?

Question

Hi,

I tried to publish my metro app in the local way, and I get a *.appx file as install file. As far as I known, this file is of zip format, so I want to unzip it and modify some file in it and then zip it to *.appx. How to do this?

The reason that I need do this is: this metro app is developed by our team , and there is another team responsible for customize the app to fulfil the requirement of customers, by replace some images, or modify some config files. So I need a way to let customizing team easy to custom this install package before we upload it to Windows Store. 

Now I can unzip it, but when I package it back in zip format, I find its size changed, so what zip format and format params should I use?