The problem with that is, that usually, if you have external numbers in a queue, you’d want to have call confirmation (press 1 to accept this call) enabled, so the callers do not end up to the external number’s personal voicemail.
If you allow putting there any string, you gain that we can put custom dial strings AND that anyone who wants to put an external number can put a local channel, like local/18880004444@cos-all
That way you can do both…