The stock TAR files contain partitions in IMG format - an all in one file system, and in this case using EXT4 formatting.
In order to create a ZIP, you need to access the files on the partition, which means finding a way to mount those IMG partitions to copy their contents, or find a piece of software to extract from them w/o mounting. Since they are EXT4, you can probably assume a Windows solution isn't available (EXT4 is Linux). Some utilities may exist to handle EXT4 on Windows, but finding one that knows EXT4 and can extract/mount an IMG file is pretty specific.
EDIT : found a utility for you - http://forum.xda-developers.com/showthread.php?t=2285831
If you can somehow get the files out of the archive, you'll need to find a tutorial on the structure of a flashable ZIP. Generally, its just folders and files but there is also a META-INF folder, scripts to install and set permissions, etc. There are likely threads here on XDA that can detail these steps.
