Yes. The end result is no different from doing it in the original build. I've got hand router jigs for this. I've even had to cut trough internal panels, but it's all doable.
You need to make sure that the top-hat wont foul something internal (like the basket/motor of the driver!).
As long as there isn't anything internal then it's as simple as cutting the hole and screwing it on. Obviously you can recess the flange if you like.
