How about something like this:

Strike of the Damned (Su): Your foe has already had the worst luck of his now cursed life, and it doesn't help when your Black Knife curses him again. When making a sneak attack against a foe who is already under the effect of any curse, you may preform a Strike of the Damned by expending 1 use of your Hexblade's Curse. If you hit, in addition to the normal damage, your foe becomes Paralyzed for 1d4+1 rounds.

You can change the effect of the strike. The main thing was that it was a cursed strike against an already cursed foe.

Also, on each of your new curses, they all say gained "at 2nd level".