I would not add any more sugar to try to "fix" anything.
Your efficiency was probably off. With a pre-boil of 1.044 and a typical boil volume reduction, without the sugar you were likely around 1.054 or so. Adding 1kg of sugar isn't going to raise the gravity anywhere close to 1.082. Without running the numbers, 1.064 sounds about right for that addition.
Ferment it. Call it a Dubbel.