Hello. I recall using sugar before, its not a horrible idea.
The concept with the final addition is that the yeast is pretty much done, but the enzymes are still around and can convert the new rice you add into sweetness. That sweetness will be in line with all the previous fermentation and be more genuine than sugar. If you can even tell the difference.
I haven't done a yodan addition in a long time. But I like dry sake.