Do the new values only need to be available in the form which has been submitted? or in every other form as well?
Usually it would be the latter, but if it's the first you could base your list box values off a repeating group in the form and then add values to the repeating group as needed.
If it's the former, you could do the same, but the submit would have to be to some central datasource like a database so that all forms reference the same set of values.