Can't install Ubuntu on USB key while keeping existing multiple partitions

Can't install Ubuntu on USB key while keeping existing multiple partitions

I've looked a bit and didn't find any solution so here's my question.
I have a USB key (16Gb) with a partition called UBUNTU and another called
DATA. Obviously, I'ld like to install Ubuntu on... UBUNTU and keep DATA
for storage when I just don't need to boot on an OS.
My problem is when I try to install on the USB key, it erases the
partitions and creates only one with the Ubuntu install.
I use this command (I'm on OSX): sudo dd
if=~/Downloads/ubuntu-13.04-desktop-i386.img of=/dev/rdisk1 bs=1m
I guess what the command says is to write on the entire device, so is
there a way to say to write on a specific partition?
Thanks, cheers!