Place the frozen berries in a medium sauce pot along with water, sugar and orange blossom water. Bring to a boil then reduce heat to low. Simmer for 10 minutes, stirring frequently, or until sugar is completely dissolved.
Remove berry mixture from the heat and strain over a fine mesh strainer into a liquid measuring cup. Cool to room temperature or store in the refrigerator until you're ready to prepare mojitos.
To prepare mojitos, fill four glasses with ice and set aside.
Place the mint leaves and lime chunks in the bottom of a cocktail shaker and muddle with the back of a wooden spoon to expel the lime juice and release the mint flavor. Add the rum and cooled berry mixture then shake well, for at least 30 seconds.
Pour mixture into each glass, 3/4 of the way full. Top each glass with club soda before garnishing with additional lime and mint to serve.