Global product categories help

so I was thinking... how would I go about bringing all the network sites categories into over arching categories so that I can then use the global product categories to show all products within a category like SPORT for instance.

I understand it would then mean the users must put a category inside a top level category kinda thing.

I understand this isn't a quick fix. Probably will code this up myself, just looking to be pointed in the right direction.

cheers guys