Preheat the oven to 350 degrees F (175 degrees C). Grease and lightly flour a 9x13-inch baking pan.

Blend cake mix plus extender, water, egg whites, and oil together in a bowl using an electric mixer until just moistened. Pour batter into the prepared pan.

Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, 32 to 26 minutes.

Meanwhile, combine strawberry preserves and pureed strawberries in a medium bowl. Stir in confectioners' sugar and lemon juice. Refrigerate until ready to use.

Transfer cake to a cooling rack and let cool to room temperature, at least 25 minutes.

Poke holes into the cake about 1 inch apart using the handle of a wooden spoon. Slowly spread the strawberry sauce over the cake, letting it fill the holes. Refrigerate until completely chilled.