Add runtime variables to UI elements which inherit from UIControl/NSControl

I've noticed that there's an option to bind runtime attribute for NSButton or any user UI element as shown in the example below. But how can I access this attribute from code for get/set operations ?

I've read that I cannot initialize attribute that isn't part of the class, but if I'll define category for NSControl which is inherited by NSButton class with runtime setter and getter to xxx, would it be possible :

@implementation NSControl (withXXX)
@dynamic xxx;

- (void)setXxx:(id)object {    
    objc_setAssociatedObject(self, @selector(xxx), object, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (id)xxx {    
    return objc_getAssociatedObject(self, @selector(xxx));
}