I mean technically you can but you won't need to bake the tart with the storebought custard in it. Store-bought custard isn't as rich and creamy as homemade and it won't set like baked custard will. It also won't level off completely flat so will make it hard to sprinkle the sugar evenly and torch it to a crackly crust. The Crème Brûlée snob in me would say to go the extra mile and make the homemade custard. It's made out of four ingredients and takes no time at all.