The easiest way is to mount the filesystem.squashfs, copy everything out into a temp directory, chroot into that directory, add all of your new entries/download all new packages, and recreate the squashfs. The commands to do so would be as follows:
mkdir /mnt/temp; mount -o loop /path/to/ISO /mnt/temp
mkdir /mnt/temp2; mount -o loop /mnt/temp/casper/filesystem.squashfs /mnt/temp2
mkdir /tmp/toEdit; rsync -avhP /mnt/temp2/* /tmp/toEdit
mount --bind /dev /tmp/toEdit/dev
mount --bind /dev/pts /tmp/toEdit/dev/pts
mount --bind /sys /tmp/toEdit/sys
mount --bind /proc /tmp/toEdit/proc
cp /etc/resolv.conf /tmp/toEdit/etc/
From here just edit like you would a normal system, then when you exit make a new squashfs and proceed like normal.
I WOULD NOT RECOMMEND a full install on the usb, if you plan on using the usb for longer than a year or two. That will eat your read/write cycles up in no time and bring the lifetime of the usb to about 1/10 of what it should be. Aside from that, don't forget to make your secondary casper-rw partition and put persistent as a kernel argument. Good Luck!