Decompression:
%mkdir initrd_fs
%cd initrd_fs
%gzip -dc /boot/initrd | cpio -id
Now you can put your modules or change init script….
Commpression :
%cd initrd_fs
%find ./ | cpio -H newc -o > /boot/new-initrd.tmp
%cd /boot
%gzip new-initrd.tmp
% mv new-initrd.tmp.gz initrd