Almost, but if I set the entire cube into smoothing group, I get incorrect normal on faces (1-st cube on the picture).
For the second cube I only included edges into a smoothing group. So edges are smooth, but don't provide a smooth transition to faces of the cube.
And the third one is what I'm looking for. Basically its like the second option, but instead of having the average normal, the vertex copies normal from a face marked as Sharp next to it (if such exists), if non are marked - then average.
There is a very quick and efficient way you can add it to PRO Builder : when you calculate smooth normal for a vertex, instead of just adding normals of triangels, multiply each of them by the area of the triangle. Edges always have smaller areas. And when you normalize to get the average normal, you'll see it will look exactly as a third cube.
I think it could be an option for each of the smoothing groups to select a method (to use area of the triangle as normal's weight or not). For my asset I made an option to manually select corners of the triangles which will use the triangle's normal for that corner's smoothed vertex normal, but I never had a reason to use it, auto is more then enough for every scenario.