What if I change Google Adsense ad codes of all pages in a network using a "proxy" or something like that? Or custom addon that changes Adsense codes; to my own Adsense code. There are about 250 computers in that network and all have own unique IP.
You appear to be asking "can I alter the AdSense code on pages served by a proxy to get clicks for my AdSense account" - this sounds more like scumware than any kind of black hat practice.
If you observe to the AdSense sign up process, you should see the requirement to (a) identify which domains your AdSense code will be used upon and (b) the clause below from the Google AdSense Program Policies:
Sites displaying Google ads may not:
Be loaded by any software that can trigger pop-ups, redirect users to unwanted websites, modify browser settings or otherwise interfere with site navigation. It is your responsibility to ensure that no ad network or affiliate uses such methods to direct traffic to pages that contain your AdSense code.
AdSense tracks which URL's users' clicks originate from - if your AdSense account is receiving clicks from other AdSense users' domains, you can expect to see your account suspended in short order.