I kind of agree with Greenmachine with a *note*. Adding more light is great but adding underpowered bulbs won't do much.. If was thinking of using something like to Bulk my grow, I'd wanna be adding 3 or 4 lol but at that point it's like you'd be far better off to just get a different more powerful light.
I have some of those same lights. I think mine are 12 watts. They are great for seedlings but you want more watts once the plants start to hit a fast growth rate. Good luck!