I have been searching for the same thing since last 2-3 days but couldnot find a reliable guide as nothing was working for me. So, i just took a trial and VIOLA!!!! its done.
There are various guides for manual extraction of such files which utilizes a dozen of softwares like 7Zip, WinRAR, MagicISO etc etc but frankly speaking none of them worked for me.
So, I thought to make a simple guide for the needy like me (with screenshots).
GUIDE STARTS HERE:
Things you need:
1. A rooted Android device.
2. sgs2toext4 (a java based applet). Download here.
3. Linux disk reader. Can be downloaded here.
1. Download and extract your stock ROM. They are available on various servers like www.sammobile.com etc.
2. You will get two files as shown below:
We only need the one having .tar.md5 extension.
3. Now, remove the .md5 from the extension of the file so that only .tar will remain.
4. Copy this file to your rooted Android device.
5. Download RootExplorer in your device and run it. Explore your sdcard in it.
6. Open it and locate your copied ROM file.
7. Open the ROM file. It will have various files but we need only two files. system.img and boot.img.
8. Long press on the file and select extract. The file will be extracted to sdcard/SpeedSoftware/Extracted/[HERE]
9. Copy these files back to your PC.
10. Start sgs2toext4.jar utility. It will open a window named Drop .img. Drag and drop your system.img on the utility.
11. You will get a new file named system.ext4.img. This is the converted file.
12. Now, download, install and run Linux Disk Reader. Select 'Mount Image' from left pane and browse for system.ext4.img. Make sure you have Raw Disk Images selected as in image below.
13. You will see a new drive created in the reader named Linux ext volume.
14. Open the drive and you will see all the system files. Select all and select Save.
15. Select a folder for saving (Preferably name it as 'system').
16. Now, you have an extracted ROM. Modify, customize and publish it.
Just Press [IMG][/IMG] to Show Your Appreciation