If your lucky and its fermented out atleast 3/4 of the sugars you might have just carbed your batch without additional sugars. Keeping the keg sealed and having the yeast eat threw the last 1/4 of sugars will leave you with a decent amount of pressure that will finish up and carb the batch without additional co2.
This is how some of the German breweries do it i think. They recirculate the spent co2 from fermentation and use the same gas to carb there batches. Some Reinheitsgebot rule of purity mumbo jumbo.
Like others have stated. You should check gravity and Since its already racked, leave it in the keg for another 2 weeks before chilling to serve