So just FYI, you are not likely going to be able to add a hopper the same day because it will likely be sold out. This is especially true for Halloween but I guess there is a possibility you could potentially do this 11/1 or 11/2.
But if you’re getting a 2-day ticket and you even remotely think you might want to hop, add it on the first day (or when you purchase your tickets). It’s the same price regardless of if you add it and use it for both days or just one day.
If you definitely want to hop, get the hopper now and make your reservation. I do not suggest a hopper on Halloween unless you plan to start in DCA and hop to DL later due to the OBB party and early closure.