the redefinition of the Core::KeyCode enumeration is not possible. The set of key codes is fixed predefined. Instead I would add 2 separate Key Press Handlers to the GUI component. The first handler could be configured to react to CursorKeys. The second handler should be configured to handle the Enter key. This is the most typical application case.
Doing this has also the nice advantage of the possibility to implement separate slot methods for the respective Key Press Handlers. For example, you could implement a slot method named onPressCursorKey and connect it to the first handler. Then you implement the method OnPressEnterKey and connect it to the second handler. Within the methods you don't need to distinguish whether the user has pressed a cursor or Enter key. In practice, you can even have individual Key Press Handler for each key e.g. Left, Right, Up and Down. This would be in total 5 handlers. There is no limitation regarding the number of handlers used within a GUI component.
The approach of having multiple Key Press Handlers does not enforce your component to have individual slot methods for each handler. You can connect all handlers to one and the same slot method. Then, however, you have to test in the slot method which handler was the sender. Accordingly you perform the associated code section.
Other also fully legitim approach is to have a single Key Press Handler configured with AnyKey as filter condition. Within the associated slot method you test whether the key code is the interesting one and if this is not the case you instruct the handler to reject this event.
I hope one of the options helps you further.