You can respin the ISOs to incude additional packages and configuration: http://community.linuxmint.com/tutorial/view/918
. But really, if you need to be able to do stuff like this why don't you just install Linux Mint to the USB thumb drive instead of using hacks like persistence? Boot the Linux Mint installation DVD / USB from another storage device, and then install it to this USB thumb drive. Make sure to install GRUB also to this device, and you are good to go. You have free persistence included, and can just as you normally would install and configure additional packages.