According to some rumors from players, this program (below) can 'copy' a 360 game and be able to be played onto the xbox360 (its illegal if done without rightful purchase of the game):
It seems it can turn a .exe to anything such as an .ISO and other formats (but must have all the necessary files to go with it).
So the question is, if a person owns Skyrim (pc) and Skyrim (360) considering that gives both programing data for the two systems, could they make a modded version and convert it to a 360 format?
*As for the modded. The game itself is already huge, mainly just equipment replacements and different clothing. Few adjustments remove and add here and there type mod.