If that is for Debian I don't know yet which tool would allow you to do it the easiest way.
To unpack an iso is easy. As root:
mount -o loop myiso.iso /a_mount_point
and then you find the squashfs file in the mount directory and you do the same with it, you mount it elsewhere and from there you will do a "cp -Ra" to your new partition (all as root). You will do a "ls" in each place where you mount your files to, so you see what it looks like.
and also see "man cp" to check what the options do.
After, that, it's Debian so I don't know. (Have you taken a look at the Debian docs to find out where to start ? Or considered asking on the relevant Debian chan ?
http://wiki.debian.org/IRC/#Debian_IRC_channels )