Here is a non-academic answer: You need to otain the specific gravity of your sugar in order to solve this one. I do not know the precise answer and, indeed, different sugars (different sources, degree of refinement, etc.) are likely to have different specific gravities along with other physical characteristics. However, if we assume a value of 2, then that will in effect give you half the volume of water (inverse of specific gravity gives you volume), or half a litre per kilo of sugar - total volume of 1.5 litres of the syrup once the sugar is added. The above value for sugar is not accurate but not wildly off either, so far as I am aware.
For an academic, and precise answer consult your nearest university chemistry department who, more likely than not, will happily give you a more accurate answer.