Not sure it that's already doable using only the object editor, but if not and you're up using triggers or code, here:
You can use an existing Missile/Projectile system (or your own). When the caster casts a spell, create the projectile and make it travel towards target. When the missile gets close enough to the target, preferably within the unit's collision radius (or use the built-in impact detection from existing Missile systems), destroy the missile, create a dummy unit that has the Slow or Cripple spell and order it to cast that spell into the target unit. Make sure also the actual Slow/Cripple spell you add to the dummy has maximum projectile speed.