All you need to do is to make a custom made cripple (set all values how you want them), set to 0 mana cost and make a dummy ability for your caster. The dummy ability should be based on an autocast-ability but should do nothing but placing a 2nd buff with the same duration as your cripple and have the mana cost you want the ability to have.
Now make a trigger that, whenever a units starts the effect of an ability (that ability being your dummy spell), creates a dummy unit at position of target unit of ability being cast, add your cripple to that unit and order it to cast cripple on target unit of ability being cast.