The cramfs filesystem is read-only, so you can't change it directly on the IA-1.
You will need the 'mkcramfs' utility from the Linux kernel source tree--I think it's under /usr/src/linux/kernel/scripts/, but you'll have to compile that specifically.
As far as I know, your best bet is to make working copies of the cramfs partitions, then make whatever changes you need to make. Rasmus has a neat script on his page that lets you mount the partitions in the Jailbait image file using the loopback device (jbmount and jbunmount). That way, you don't have to dump the image to a CF card before mounting the individual partitions.
When you're done changing things around, use 'mkcramfs' to rebuild the relevant cramfs images, then somehow insert them back into the Jailbait image (this part I'm a little hazy on--I've been trying to rework the Jailbait image in my spare time, and I haven't got far enough for mkcramfs).