TechUSB uses SysLinux as its bootloader, and this article will show you how to add your own ISOs to the Boot Menu.
Example with Clonezilla:
After creating your TechUSB drive, open it up and view the files. You should see the following:
Open the "boot" folder, and create a new folder called "ISOs":
Place your custom ISO file in that folder:
Now navigate back to the "boot" folder and go to the "isolinux" folder instead:
Using your favorite text editor, open up the file "knoppix.menu" for editing:
Scroll to the bottom, and add the following:
MENU BEGIN customisos
MENU TITLE Custom ISOs
MENU LABEL clonezilla-live-2.4.2-10-amd64.iso
APPEND --config-file="map /boot/ISOs/clonezilla-live-2.4.2-10-amd64.iso (0xff);map --hook;chainloader (0xff)"
Save all of your changes, and properly eject the drive. You should now see the option to select your new ISO at the bottom of the TechUSB boot menu.
For more documentation on how to add items to the boot menu, consult the syslinux documentation here: http://www.syslinux.org/wiki/index.php/SYSLINUX#What_is_SYSLINUX.3F