Put the graham cracker crumbs and pumpkin pie filling in a large bowl and mash together until a thick dough forms. Scoop the mixture by level tablespoons and place on a parchment-lined baking sheet. Roll each tablespoon of dough into a ball and freeze until solid, about 1 hour. Meanwhile, microwave the melting candy in 30-second intervals, stirring in between each, until melted and smooth. Dip each ball into the melted candy and place on the prepared baking sheet using two forks. Garnish each truffle with a pinch of grated nutmeg. Let stand until firm, about 30 minutes.