What you have to do is get a "dummy" edge right up near the corner, the rounding Poser does is just a function of how far it is to the next vertice. RDS default cube has only 1 face per side, try subdividing it about 4 times and you will see what I mean. In 3DS you can cut polygons to do this, I don't remember RDS having a easy wau to do this though- if you do partial dubdividing RDS will make "bad" mesh, with polygons that will open holes if you do much with them.