It will work but it will thin out your beer and add unpleasant ethanol (alcohol/"boozy") flavors to your beer. I personally would suggest adding a yeast nutrient and a second pack of yeast, both available at your lhbs. The nutrient will help your yeast achieve full attenuation and the second pack will ensure you have enough cells to rip through the fermentation. Just add the nutrient dry in the last 10 or so mins of your boil. If you are trying to raise your OG, thereby raising your final ABV, without changing your attenuation then I would advise adding more extract instead of adding sugar. It still will help to add more yeast and a nutrient. Keep in mind what has been said above, this will then throw off your hop utilization/ibu so you then will have to add more hops to keep the final profile similar to the intended beer just higher alcohol.
I know you bought a kit and probably don't want to mess with it but as you move forward keep in mind what extract you are using. The darker the extract, the lower the fermentability. I only use Briess extract (even in my commercial brewery) and typically use their Golden Light DME (think Light DME) when I do. They advertise 75% fermentability however I typically achieve 80-85%, and that is using s-04 which typically has lower attenuation. I have heard of many people achieving 90%+ attenuation when using their Pilsen Light (think Extra Light) extract. I rarely use Muntons but anytime I have I've always had terrible attenuation with even their lightest of extract resulting in unusually low alcohol, too much residual sweetness, and too much body.
