Completely fermentable sugar will lower the FG, as EtOH has a lower specific gravity than water; this why a table wine with a 1.090 SG has a FG usually around 0.990. It won't matter when you add the simple sugar, the sugars will get used eventually anyway.
Assuming a mostly malt wort, so there's plenty of FAN, vitamins, etc, what matters is will the amount of potential alcohol exceed what will kill your yeast (varies by strain) and does yeast make an enzyme that can eat that sugar. Since Belgian candy sugar is sucrose already broken down into glucose (aka dextrose) and fructose, the answer to that is "yes".
This is why lactose can add body; Saccharomyces cerviseae yeast don't digest it.