Lightly coat the inside of a 3-1/2- or 4-quart slow cooker with cooking spray; set aside.
In a medium bowl stir together flour, granulated sugar, baking powder, cinnamon, and salt. Add milk and melted butter; stir just until combined. Stir in 1/2 cup pecans and currants. Spread batter evenly in the prepared cooker.
In a medium saucepan combine the water, orange peel, orange juice, brown sugar, and 1 tablespoon butter. Bring to boiling, stirring to dissolve brown sugar; reduce heat. Boil gently, uncovered, for 2 minutes. Carefully pour over mixture in cooker.
Cover and cook on low-heat setting for 4-1/2 to 5 hours. Remove liner from cooker, if possible, or turn off cooker. Let stand, uncovered, for 45 minutes to cool slightly.
To serve, spoon pudding cake into dessert dishes. Top with caramel topping and additional pecans and, if desired, whipped cream.