adding new property to an object exposed to scope in angularjs

Just add the array to the object.

$scope.myobj = {};

...

// sometime later
var numbers = [1,2,3];
$scope.myobj.numbers = numbers;

EDIT:

To answer your question about scope in a service. Scope is not accessible in a service. Typically you ask your service for something IE data. But your service can do anything, like add 2 numbers, or in your case create an array of something that you need to attach to your object.

module.service('MyService', function() {
    this.add = function(number1, number2) {
      return number1 + number2;
    }

    this.createMyArray = function() {
       // pass in whatever you need to this service in order
       // to create the array you need.

       // example of just returning a hard coded array
       return [1,2,3];
    }
});

Then you can inject your service into your controller which has the scope you want to modify.

app.controller('MyController', function($scope, MyService) {

    $scope.add = function(number1, number2) {
        // Lets use our service to add these numbers and
        // assign result to scope variable
        $scope.answer = MyService.add(number1, number2);
    }

    $scope.myobj = {};
    $scope.makeArray = function() {
        // lets use our service again to create an array
        // this time lets add it to a scope object that already exists.
        $scope.myobj.numbers = MyService.createMyArray();
    }
});

A lot of times services are used to grab/update things from a server, so you will see a lot of examples that make http get/post/put/delete calls. Then from your controller(s) you can use those services to grab data. But again you are not limited to that, your service can simple just hold some static data or helper functions too.