These are only used where the drain is a long way from any available place where you can vent normally, and it still needs to have a real vent somewhere in the house! You often see them in places like an island sink where you don't have any way to hide a vent line up fromthat location. I doubt code would allow them in your situation. You can move the existing vent out of the middle of the new room. You should also use a new vent (or connect to the existing one - easier to do a new one since you are right there near the roof, or you run into 'wet venting' which can get messy) for the new bathroom. Connecting the drain into the existing one also requires some work. I'm not a pro, so can't give you the details. Basically, you don't want to be dumping stuff down the vent line from below (wet venting) without following all of the rules, or you run into problems.