That's a big room, and probably with lots of windows to use one duct to service it. And, the bathroom would likely not cool as well. With the added floorspace, you should probably redo the manual-j (heat loss cal) for your house and see first, if the thing can handle the additional load. Sometimes, yes, sometimes no. The only good thing about adding more load is that the thing will run longer, and if it wasn't already, it will pull more moisture out of the air, but may no longer reach the desired temp. You'd probably be better to tap off the main with a new run, than tapping off the existing single run to branch into that room.