It's amazing how easily a sand pile in the back yard can migrate to a beach area in the front yard about 50 yards away. It doesn't hurt that the neighbor's children play on said beach.
I have no idea how strict they are about granting you the ability to add sand, but once you've notified them be prepared to either live with their answer or to knowingly violate their decision. Keep in mind that as Dock and Barge states, you can only legally replenish the beach every five years which would probably leave most individual beaches looking a lot worse for the wear.