The info's around here somewhere. Run a search on "backup game channels", "crap", and "loadstructor". The first will lead you to a thread by sociallyantisocial and has wads available for download, the second is a program for creating them yourself - look for the guide written by wiijohn, and the third is another program for creating them.
There's also Dogeggs' Guide To Creating Channels with WADder.
WBFS Manager needs to have channel creation enabled in options. More on this can be found in the online help file for wbfs manager.