No, it is not possible. When you go into the NBT file database, you can change the level, by inserting the level you want. The way it is accessed is that it puts the level like this: "enchantment.level.[level]". If it recognizes the level and the enchantment as useable, it converts it to something like "knockback V". If you put a different letter in, it will default to no enchantment. This includes the "-" character, so if you put -1 in it will come out as "enchantment.level.-1" which it cannot recognize, therefore nothing special happens when you use the sword.
It would be pretty cool, though! :D