Well, if the icns already exists, it is much more rational to place it yourself in the bundle with a copyFile step, than to drag it over the icon editor to add more noise to the signal.
BTW I am not sure this editor would be astute enough to assign the proper size to each box.
I usually start with a 1024x1204 picture, and reduce with gaussian sharp in Photoshop to get the best result at smaller point sizes, especially 32x32 and 16x16 where the Xojo editor would have a very poor rendition. For some apps when reducing to 16x16 would not do, I even changed the design of the icon.