Convenience for Custom List Values

First: the Custom List Values for the tags are a very valuable feature to me
And in the meantime there are some suggestions for better handling of
long existing lists to import.
But one feature that would obviously be very helpful and I think easy to implement
was overlooked:

Why not have an additional entry in the tag dropdown menu to put the existing value
in the Custom list. So it could be filled easily while tagging and at least I would
not be needing to import any lists.