Thaw frozen pineapple on a counter for 15 minutes in summer (if your house doesn't have AC) or 20 minutes in colder months. You are looking for defrosted semi-soft outer layer but still frozen inside fruit.
In a food processor, add pineapple and process on High speed, pausing and scraping the walls with spatula and adding 2 tablespoons of cold water at a time to help pineapple pieces move around. As long as fruit is blending do not rush to add more water.
Serve immediately or freeze leftovers for a few hours but not longer.
Notes
Freeze leftovers: Transfer mixture to freezer-safe Ziploc bag, spread into a thin layer within the bag, and then freeze flat for up to a week. When ready to use, break into pieces, then transfer to your food processor and blend back into the correct consistency. Add a little water or milk as needed.