Converting Linux ISO images to USB is a huge topic because each distro CD has its own boot system. The method changes for different versions of CentOS, and even between the flavors (rescue, live, install) of the same version. Thus it can be challenging to create a USB flash drive that will both do a rescue/live boot and an install.
If you just want a rescue boot, then I'd go with any Linux distro and create a USB flash drive that lets you boot either i386 or amd64 (defaulting to your preference) and has mdadm, LVM, and any other mission-critical tools. Note that an i386/amd64 mismatch will cause chroot to fail with an obscure message such as "/bin/sh can't be found".
If you really want an install option, you can try this website's techniques, I've had varying success. What's good about this system is that you end up with menus that expose the various boot systems for each distro. What's bad about it is that it fails you won't know why.
The best resource I've found for CentOS is this one:
They tell you how to do what you want (you create a separate partition with the CD Images that get mounted as loop devices so the installer can find them). But it's a bit of a PITA and may or may not actually work. You might have better luck with a Net Install image and a using a kickstart to pre-load your favorite mirror.
Finally, and inevitably off-topic, I like this site because they actually tell you how to do make a bootable UEFI USB flash drive from scratch (albeit on a Windows 7 system). The technique can be generalized provided that you can figure out the ISO boot mechanism for your distro.