The rootfs.gz is a compressed archive of the entire file-system and I would think a GZip utility is available for Windows to de-compress it. (I think it is actually compressed with CPIO and then GZip, I'm not sure!)
I should imagine that it would just be a matter of expanding the rootfs.gz file in to a directory while in Windows, adding the files to the directory structure, then re-compressing it. The customised ISO could then be re-made with the new rootfs.gz file included.
I've not tried it, but with some experimentation and reading you may find a procedure that works.
I try with gzip for Windows, 7-zip and Peazip, but this program unpack rootfs but don't in to structure folder. It only file of di 90 mb. Search that cpio for Windows but dont function.