I don't know too much, but I do know that there are 7 .sig files that are required by the Xbox to recognize the savegame as legit. KSE will create 6 of the 7 .sig files. Unfortunately I don't know how to get the 7th one (SAVE_HEADER.sig). This means that you have to start with an Xbox save to begin with that has a legitimate SAVE_HEADER.sig. When you move the Xbox save to the PC, you can follow the KSE instructions to modify it (limited modifications), and then move it back to the Xbox using the original SAVE_HEADER.sig and the 6 new .sig files. If you don't edit too extensively, you should be able to use the same SAVE_HEADER.sig.
Of course, all this is moot if you don't have the SAVE_HEADER.sig in the first place.
(Also, I don't own an Xbox, so I can't give you details on how to move files back and forth to the PC.)