Yes you can preheat the oven to 200 F and then turn the oven off but place the Pizza with the cardboard box inside the oven for awhile and the heat from the oven will keep the pizza warm enough in the cardboard box.
However if you leave the oven on it may eventually catch the cardboard box on fire or at least burn it somewhat should you forget to turn the oven off.
So yes you can put the cardboard box and pizza in the oven on 200 F for a little while but after around 30 minutes or so turn the oven off and let the heat in the oven itself keep the pizza warm.