ReactionCore.Schemas.userPrefs = new SimpleSchema([
  ReactionCore.Schemas.Accounts, {
    defaultGame: {
      type: String,
      label: "Default Game",
      optional: true
    }
  }
]);

// And it's called from a method using:
    var userId = Meteor.userId();
    ReactionCore.Collections.Accounts.update({_id: userId}, {$set: {'defaultGame': 'plz work'}});