Preheat oven to 350 degrees F (175 degrees C). Grease 2 (9-inch) round cake pans.
Press 2 cups of salmonberries through a metal mesh strainer into a bowl using the back of a large spoon; discard contents of strainer. Pour salmonberry pulp and juice (it will be about 1 1/4 cups) from bowl into a 2-cup measuring cup; pour in enough water to bring the pulp and juice to 2 cups. Stir well.
Place cake mix, eggs, vegetable oil, and 1 cup salmonberry mixture in a bowl; blend with an electric mixer on low speed until moistened, about 30 seconds. Beat cake mixture at medium speed for 2 minutes. Fold remaining 1 1/2 cups whole salmonberries into cake mixture; pour evenly into prepared pans.
Bake in the preheated oven until a toothpick inserted near the center comes out clean, about 30 minutes.
While cake is baking, cook remaining 1 cup salmonberry mixture, remaining 1/2 cup whole salmonberries, and sugar in a saucepan over medium high heat, stirring constantly, until mixture begins to simmer; remove from heat.
To assemble: While cake is still warm, gently poke holes all over the top of the cake layers with a fork. Pour half the cooled salmonberry mixture over each cake layer; allow to cool completely. Allow to cool completely.
