I do that all the time! The only thing I see that I would do differently is to not pack the dryhops into the bag so tightly. If you're going to use a bag, make sure the hops are in there very loosely, so that the beer can permeate the hops inside. Use two or three bags, if needed, so that the hops are loose in the bag. Otherwise, that's what I do. I ferment for 7-10 days or until the beer has been done a few days and is clear (or clearing well). Then gently add the hops, let sit 5-7 days, and package. That's it!