I doubt that the Impale damage is actually added to your hit. My guess is that it's its own separate effect dealt as secondary damage.
The damage from impales on the enemy is separate from the hit triggering them, and is reflected damage.
If multiple impales are on the enemy, they take a single reflected damage hit with the stored damage of all the impales combined, rather than several smaller ones, and then all the impales tick down one "use" (being removed if all 5 are used up).