redNAND is a way to redirect NAND access to SD card's hidden partition.
You can't create a redNAND on USB.
You also don't really need redNAND if all you want is games on USB and signature patch, as you can use standard Custom firmware with signature patch without NAND redirection too (sysNAND sign patch)
if you want to install games on SD but not on NAND or USB, then you will need redNAND on SD.
games on sysNAND : only sysNAND
games on redNAND : only redNAND
games on USB : can be launched from both sysNAND and redNAND
games with wrong signature requires signature patch from a CFW (redNAND or sysNAND CFW)