beep beep muwhaha i know the answer
first you know the skin file? the one that has like model_default and its possible to open into a word document well you need to set all the bits that say xemnas to kyle (which is the default skin for both sp and mp)please be careful though when saving the skin file to just save it and make sure to say yes to it saving even though it might not recognise a skin file cause otherwise it WILL NOT work.
then you need to as graan said change the directory you put it in to kyle so it now looks like this
make sure you have all the parts of the skin you need including the model then well pack it up into a pk3 and your done
the same can be done for almost any of the original skin's you just need to find the director and original name ect change your new skin to match that and you should be fine
From Ralen Animus