Several ways. To make the instant death feature for an item, assuming you're using KT, you'll select On Hit for the Properties and then select a DC of 100 for the save if you want to make it a guaranteed kill.
To change the amount of damage dealt by the weapon, you would select Damage Bonus or Damage Penalty for the Properties and then select the amount to modify by. I wish I had KT with me now, I'm going from memory now.
As for modifying a Force Power from an item, not really doable. The amount of damage dealt by a Force power is governed by a script, not something an item can change. You could always change the Damage Resistance or Damage Immunity of an item, but you can't directly change the damage of the power unless you change the script.
- Star Admiral