Yes, you can definitely use frozen broccoli florets instead of the fresh broccoli. If you want to use frozen broccoli however, I would just steam it in the microwave rather than adding it to the pasta at the end as instructed in the recipe. Or, you can prepare it how you usually like to - pan-fried, baked, etc. Cooking it separately will allow the pasta to finish cooking without bringing the temperature of the water down too much. However you choose to cook it, try not to overcook it so it will hold its shape in the salad. After cooking the broccoli, let it cool down for a few minutes, then add it with the rest of the ingredients when ready to mix the pasta salad.