Place the plantains in water to cover in a large pot. Bring to a boil, then reduce heat and simmer until tender, 15 minutes. Drain and mash.
In a small saucepan over low heat, heat the refried beans. Stir in the sugar and salt. Remove from heat.
To make the rellenitos, form a palm-sized amount of mashed plantains into a ball; flatten and place about a teaspoon of the bean mixture in the middle; then mold the sides of the plantain around the beans, making an egg-shaped ball.
Heat oil in a deep-fryer or large skillet to 375 degrees F (190 degrees C), or until a small amount of batter sizzles and browns in the oil. Fry rellenitos in oil until browned. Drain on paper towels.