Thanks to both of you for your input. The elusive goal I would really love to get hold of is to give people a button by which they could actually 'unlock' the semi-colon-delimited Value List which appears in the field properties window as the row source, add an item to it, and then have it re-sorted in proper order for the next time someone clicks the dropdown arrow for a given combo box, as if they had gone into design view and done it there themselves.
What I sometimes do as a substitute is simply store all available input choices in a separate table, which generally only requires one field, and query it as the row source. Instead of having a regular label next to the combo box, I make it a button that takes you to the table where you can add options at will. This works, but I think there must be a way to do the other.