Line a flat baking pan with foil and set potatoes in the pan.
Preheat the oven to 350 degrees.
Scrub potatoes and bake them until tender, about 45 minutes.
Let cool, remove flesh and discard skins.
Spray a shallow baking dish with nonstick vegetable oil.
Mash the potatoes until fluffy and pile into the dish.
Spoon the pineapple over the potatoes.
Cover and refrigerate until ready to bake.
Bring to room temperature, then uncover and heat in a 350 to 375 degree oven until hot, about 30 minutes.