If the type of the wire changes so that it is carrying additional information, it becomes a language construct. I was looking at it from that point of view, but, you are correct, the feature as requested is just an editor feature.
If you're really doing this often and want to save time, you could also write a Quick Drop shortcut to do this which you invoke on the case structure. It would have to trace the input to the selector back to the control (assuming it is a combo box control) past tunnels, etc., and then get the list of values from the control and add missing cases to the structure.
Or you could automate the process you just described. Either way, not code I would want to write, but it could help you if you do this a lot.
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