Yep! You can certainly use frozen berries instead of fresh fruit. I often recommend letting frozen berries thaw first to keep your mixture from being too wet. Since I find this delicious apple blueberry crumble with oats can be a little softer than other crumbles, it would be helpful to thaw the berries first. If you don't, it will still taste great, but the fruit mixture may be a bit thinner than you're expecting.