addign 2kg of sugar with 1 tin of malt extract in 25 litres would likely result in pretty awful beer.
sugar is 100% fermentable so it will turn compeltely into alcohol. That is until the alcohol kills off the yeast, and you are left with residual sugar.
You will end up with a high strength, watery, thin, beer flavoured alcohol.
That said, I have not tried this (nor do I intend to!) but there is nothing stopping you from doing this and seeing what you think of it. However, compare that, to a premium kit brewed to instructions, and i guarantee you will notice the difference!