If you've got the Togepi Egg and it hatches it will stay the same nature from the first time you received it from the Kimono Girl.
You can however SR for the Egg to get a Modest. Before you receive the egg from the Kimono Girl save your game. Hatch the egg and don't save during the process of egg hatching. Once you have hatched the Togepi if its nature is not Modest reset your game and you should be back just before obtaining the egg. Keep doing this until you get a Modest Togepi.
If you breed your soon to hatch Togepi with Extrasensory then its offspring will inherit the move as long as Togepi still has move on it.