i think there is a channel for the USB loader or u can just use a forwarder channel for it.
go here and look for "There are three ways to install and use USB Loader GX"
and look at the ways 2 and 3. Should be what ur looking for.
I dont think u can make channel for a game that is on ur USB HDD.
but usb channel saves u the trip to homebrew ch and runing it from there.