In a food processor with knife blade attached, pulse frozen pineapple until it resembles finely shaved ice, occasionally scraping down side of bowl with rubber spatula. (If pineapple is not finely shaved, sorbet will not be smooth.).
Add yogurt and sugar to pineapple and blend until mixture is smooth and creamy, occasionally scraping down side of bowl with rubber spatula.
Serve immediately for a creamy texture or, freeze and serve later for a firmer texture. If made ahead, remove from freezer 5-10 minutes before serving.
Scoop sorbet into dessert bowls or martini glasses. Garnish with fresh berries if desired.