I have used liveusb-creator with some success (particularly with Fedora images.)
Notice the "persistent storage" option on the window; if you set it to a value, then you will find that much persistent space available to your usb image.
It's been a few months since I used one of these images, but I seem to remember that you could add new packages to them that persist over reboots, and the webpage I reference above also indicates that.
ML/NJ