You might as well do that using an NSMenuDelegate method.
Actually in this way you can also keep the bindings for getting the NSPopUpButton content objects (in your case from the NSArrayController bound to the CoreData stack).
1) Set an object as delegate for the NSPopUpButton internal menu, you can do that in the Interface Builder by drilling down the NSPopUpButton to reveal its internal menu. Select it and then set its delegate in the Connections Inspector panel to the object you have designated to this task. As such delegate you might for example provide the same ViewController object which manages the view where the NSPopUpButton exists. You'll then need to have the object provided as delegate adhere to the NSMenuDelegate informal protocol.
2) Implement the NSMenuDelegate method menuNeedsUpdate: there you'll add the NSmenuItem(s) (and eventually separators) you want to provide in addition to those already fetched by the NSPopButton's bindings. An example code would be:
#pragma mark NSMenuDelegate
- (void)menuNeedsUpdate:(NSMenu *)menu {
if ([_thePopUpButton menu] == menu && ![[menu itemArray] containsObject:_editMenuItem]) {
[menu addItem:[NSMenuItem separatorItem]];
[menu addItem:_editMenuItem];
}
}
In this example the _editMenuItem is an NSMenuItem property provided by the object implementing this NSMenuDelegate method. Eventually it could be something as this:
_editMenuItem = [[NSMenuItem alloc] initWithTitle:@"Edit…" action:@selector(openEditPopUpMenuVC:) keyEquivalent:@""];
// Eventually also set the target for the action: where the selector is implemented.
_editMenuItem.target = self;
You'll then implement the method openEditPopUpMenuVC: to present to the user the view responsible for editing the content of the popUpButton (in your case the CoreData objects provided via bindings).
The only problem I haven't yet solved with this approach is that when getting back from the view where the edit happens, the NSPopUpButton will have the new item "Edit…" selected, rather than another "valid" one, which is very inconvenient.