Slime spawning chunks (like biomes) are generated on the fly using the seed of the map. With mcedit/nbtedit you can change the seed. With mcedit, load the world, hold ctrl, press I, and change the seed. With nbtedit, load the level.dat file, and change the "seed" entry. If you use nbtedit, you can edit enchantments, xp, xplevel (they're different), the player's invintory, ect, ect. Have you seen the enchanted sign in vechs' "mini hostile: ragequit holidays"? You can do that with nbtedit.