From what i have tried personally you can only edit 8 kits using the Scan method in Pro Evo 2008 itself. Even if you could edit more you cant really add custom shorts, socks or backs of the kits.
Their is another technique which im experimenting with atm importing them in using a program called GGS,
e.g. their is a bin file called something like FranceKit.bin or thereabouts it all it contains is 2 images which are basically a blown out version of the kit 2D style. and you can replace those with other 2 blown out versions, i personally tried the Aston Villa kit i found and it imports perfectly.
the only catch is their are lots of Unnamed_kit.bin files which are basically blank kits the problem im having is finding out which bin files are for which teams then its just case of importing them one by one, i'd imagine once thats all done and figured out someone will just create a patch for it.
hope ive helped.