Depending on the type of tree and your location, drilling holes and filling them with mushroom spore plugs might also work. The mushrooms will help to break down the stump, and you can harvest mushrooms from it in the meantime.
If you have an auger, drill a bunch of holes...I mean saturate the stump with holes, fill those holes and cover the stump with manure. It will still take a long time to rot, but this will speed it up a lot.
High nitrogen fertilizer in the already mentioned, drilled holes.
The StumpBGone stuff you can get at greenhouse supply stores is usually nothing but potassium nitrate, so if you've already got a high nitrogen fertilizer, you can just use that. Keeping the stump damp will help fungus fluorish in the high nitrogen environment.
Sugar is just carbon, hydrogen and oxygen; that'll get you yeasts, but no real lignin eaters, which is what you want.
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.