I can tell you how to extract the initramfs image, but I don't think anybody has had success in inserting an altered image into the stock kernel, so it's only of use in building your own kernel. You need to search for a gzip header in the zImage, it is a sequence of three bytes: 1F8B08. Gunzip this and all data following it to get the Image file. Search that for another gzip header, which is the start of the initramfs, in the form of a gzipped cpio archive. You will need to unpack this if you want to modify it. Be aware that it contains absolute pathnames and that cpio will cheerfully and silently extract these to the root of your filesystem if to don't explicitly tell it not to.
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