You haven't said whether the stumps are still live or not - if they've recently been cut down and not treated with a stump killer, they will regrow, so that's the first thing to do. After that, there's not much you can do to speed things up other than leave fungal activity to do its thing, the majority of which goes on underground, where you can't see it happening. Using sugar to aid this process isn't something I've heard of, but keeping the stumps damp will. Don't soak the stumps regularly before the stump killer has had a chance to work, you don't want to wash that off. You could use organic materials to keep them more moist - grass clippings, etc.