Hi & welcome,
I haven't done this but I'd guess its feasible.
You could try adding a bit at a time over the next few days until hopefully you hit the right level of sweetness before you plan to bottle. Add say 50 or 100g to some boiling water, say at least a cupful, carefully pour it into your fermenter and stir gently with a well- sanitised stirrer- thingamy. Not sure if I'd actually boil lactose, but you want it to be spankingly clean, as does your handling with the lid/ film off the fermernter.
They do ferment down pretty dry the straight kits, a few similar reports around in here somewhere and various methods to counteract it, although for my upcoming few batches I haven't really decided what to do about it. Definately won't be using the champagne yeast they come with, its a shocker.