I think the point was to enable the clothing for other categories, without just making a copy in bodyshop and getting a bloated duplicate.
I hope someone has a tutorial. I know how to do it, but I'm not in the mood for writing a tutorial, and I'm not much good at explaining anyway.
Oh, and it's the Property Sets, not Material Definitions you need to edit. My bad...
Edit:I actually found something! Two tutorials by Windkeeper, the
first one is not actually related, but it does tell you how to find the original bits in SimPE. You just need the Property Sets of the clothes you are changing.
Then use the second part of
this tutorial, the bit that tells you how to edit a package directly, for the category numbers.