In a few favored locations around the country, fresh fruit and berries are available through most of the year. Everywhere else, the options most of the year are mediocre fresh fruit trucked from thousands of miles away, or frozen fruit. Frozen fruit is often the better option, because it's harvested at its ripest and processed immediately to preserve the best flavor. Adapting your cake recipes to frozen fruit is relatively straightforward.
A few recipes call for the fruit to be used as a layer or swirl within the cake. Usually the fruit is at least partially cooked beforehand, and the cooking juices thickened to make a flavorful sauce. You can substitute frozen fruit for the fresh fruit called for in your recipe, but it's important to note that the frozen fruit will shrink more during cooking then fresh fruit and will release more juice. You might need to cook slightly more fruit, and concentrate the juice by simmering it down to half its original volume before thickening it. Once the fruit is cooked and cooled, it can be used in the same way as fresh fruit.
Most cakes incorporating fruit or berries call for them to be stirred into the batter before it goes into the pans. Large frozen fruit such as peaches or mango slices should be cut into bite-sized pieces, but berries can usually be used whole. It's best to bake the fruit while it's still frozen, because if thawed it will release its juices into the batter and make it watery. Deeply colored berries will also color the batter if they're thawed, which is usually undesirable.
A Few Tips
The thinner and lighter your cake batter is, the more likely it is that your fruit will sink to the bottom during baking. To counter this, cut your fruit into smaller pieces for thin-battered recipes. You can also toss the frozen fruit with flour, which will help it remain in place. If you're using especially fragile fruit, such as frozen raspberries, it's best to push them gently into the batter after it's in your pans rather than stirring them into the batter. Cakes leavened with baking soda tend to give blueberries an odd green tinge, so it's best to use baking powder for blueberry cakes.
The simplest cakes to make with frozen fruit are upside-down cakes, which bake on top of a layer of fruit. Frozen apples, pineapples, pears and other fruits with moderate amounts of juice can be used without thawing. Grease the bottom of your pan liberally, mix your sugar with cornstarch or flour to thicken the juices, and proceed as you would with fresh fruit. For juicier fruits such as peaches and berries it's best to thaw them first, draining and saving the juices. Thicken the juice separately, then pour it over the fruit and proceed as you would for fresh fruit.