Yes you can add it but it is more about supplementing the plants carbohydrates as they get all used up during flowering. i have heard a lot of people using molasses but there are a lot of better sources of carbs for your plant.
I use Carbo load made by advanced nutrients. This exactly matches the plants carbohydrates and eliminates any waste from using things such as molasses. I start using this product about a week or two into flowering, and continue until the final flush.
This serves two pursoses for me as I also utilize benefical bactaria and fungi in my grow to help fight disease and stimulate explosive root growth. This carbo load also feeds them so that they thrive because plant carbohydrate is what they eat.
Here is a pic of the carbo load.