You have to:
-install genromfs
sudo apt-get install genromfs
- create two directories
mkdir romroot
mkdir rwroot
-mount the rootfs to romroot
sudo mount -t romfs -o loop rootfs romroot
-copy WITH SU RIGHTS (it won't work as a common user) the content of the mounted filesystem
sudo cp -r romroot ./rwroot
-unmount the fs
sudo umount romroot
-modify the rwroot folder as you like
-create the new rootfs with genromfs
cd ./rwroot/romroot
genromfs -f rootfs