Wouldn't be that hard. The hardest part is that you would have to add those animations to the target. Also, you would have to make sure that the weapon doesn't get too "friendly" with the target - remember that the first person mesh is actually very small so it doesn't intersect with walls, so you just have to make sure it looks like your knife is cutting throat, when it's really far from the throat. Of course, if the target is a player, getting the target to see it happen would be a bit harder... then you would probably have to have a special "knife attack" animation for the whole player model.
Eater.