How to add attribute to NSManagedObject class?

I have a core data class that I created from my model file.

Just like this:

And then I used the editor to generate a sub class of NSManagedObject for that entity like this:

And this is the class that was generated for me:

But now I want to add another attribute to my Target entity...obviously I cant just add an attribute to the class, so what is the right way to do so? lets say I want to add an NSString property like: @property (nonatomic, retain) NSString * someProperty;

Thanksss