well not sure if its possible but if it is you definitely would need to have the xdk (xbox development kit) which unless you are an official xbox developer would be illegal

your best bet is to find a skin that has the all the individual png files in uncompressed texture form- such as KCJuggalo on allxboxskins, and edit/replace the png's individually
XBMC can handle eithe all the png's compressed in .xpr texture format or in the raw png's (assuming the match up to the filenames used in the various skin controlling xml files), or a combination of both
HTH