Mocking method results

I'm trying to find a way to fake the result of a method called from within another method.

I have a "LoadData" method which calls a separate helper to get some data and then it will transform it (I'm interested in testing the transformed result).

So I have code like this:

public class MyClass(){
  public void LoadData(){
    SomeProperty = Helper.GetSomeData();
 }
 public object SomeProperty {get;set;}
}

I want to have a known result from the Helper.GetSomeData() method. Can I use a mocking framework (I've got fairly limited experience with Rhino Mocks but am open to anything) to force an expected result? If so, how?

*Edit - yeah as expected I couldn't achieve the hack I wanted, I'll have to work out a better way to set up the data.