No I mean, you need something like Microsoft Excel Editor to edit .slk files, an MPQ editor to open WarCraft III's MPQ's and extract Terrain.slk, and modify that Terrain.slk, and stick it back into your WC3's MPQ. Next time you open up your World Editor, you'll see the new tiles. Or, in theory anyway.
It's of course a bit more complicated then that, but that's the basis of it.