I presume it can be done with Oztrail, but when we ordered our Camper Trailer we also bought all the walls to enclose the annex plus a kids room which attaches to the drawbar side of the trailer. So in short it gives another room the same size as your tent area.
Pros of the kids room is it gives heaps of room for everyone to spread out especially on those days when the weather isn’t that great. When we camp for 10 to 14 days we love the extra room, but I refuse to put it on unless we are camping a minimum of 4 nights.
Cons to the kids room is it adds another 40 minutes to the set up/pull down time and the extra canvas/pols/ropes isn’t light