Sure. The problem (for some users) is to get them placed accurately enough, relative to the headers, for a manufactured board to actually fit where intended.
I don’t use any of the methods you suggest. Instead, I edit the source svg file directly, and set coordinate values that way. Once I have the offsets from someplace else. It is not the UNO shape, but that is what I did for the pcb shape to create a part for a shield board. That worked, once I figured out the limitations and work arounds for Fritzing not handling some interior cutouts in the board shape.