repack initrd

unpack:
file initrd
gzip compressed data, from Unix, last modified: Tue Mar 11 10:56:38 2014, max compression
mv initrd initrd.gz
gunzip initrd.gz

Now we got initrd file, which is cpio archive, unpack it:
cpio -id < initrd

and you could do with it what do you want,

after that we have to pack it again
find ./ |cpio -H newc -o > /tmp/initrd.cpio
gzip < /tmp/initrd.cpio > /tmp/initrd

and we got new initrd !