Most Helpful Guy
You could use powdered sugar on top, or serve it with sour cream and berries to mask the unsugared pie. If you have time, I'd make another, though. pie is better when served right from the oven anyway.
Most Helpful Girl
Did you want the sugar to be added to the crust or to the filling?
If you wanted the sugar in the crust, I would suggest using an egg wash. Beat a raw egg, mix in some sugar, and use that to glaze the crust when it has 20 minutes or so left to bake. You can also sprinkle some additional sugar on top after you glaze the pie- it will stick to the egg wash.
If you wanted the sugar in the filling, you have fewer options. You can try to remove the top of the pie crust (the "lid" if you will) in order to mix some sugar into the filling, but the crust may never stick on properly again. You can simply serve the pie with sugar so that people can sweeten their slices to their own desired level of sweetness. Or you can garnish each slice with sugar syrup or sweetened whipped cream to add more sweetness.