Is your hitbox also an object in the sequence?
This is doable, but probably not the most efficient way to handle it. If the hitbox doesn't move at all relative to the player, I would just spawn the hit box object directly. If the hitbox does move relative to the player and you want to use the sequence to animate it, I would only animate the hitbox object using the sequence and set the sequence origin to be the same as the player's sprite's origin, then create and destroy the sequence the same way you would the hitbox, without messing with the player or the player's sprite (which would be animated in the traditional way).
Mathroo has a tutorial on instance overriding which could be useful if you do want to try your original idea though: