The hardest part of adding a new storage door will be framing the opening with wood, so as to have something to secure new door to. If you have to cut through a stud to install door, it becomes more complicated framing opening, as you now have to cut stud short inside wall to allow new top/bottom framing between exist studs. Also at sides of door you will have to add framing too. And if you have a rounded corner door you will need blocking at corners. Easiest thing might be get a door that will fit between two studs.