Is there anyway to add data to a list box using Campaign Builder

I’d like to add data to a ist box using Campaign Builder. It seems logical that if I set a value that is in the listbox, and set it not to overwrite existing data, that it would just select the new item and leave the other items that were selected. However, what it seems to do is just leave the items that were selected alone and not select the new data. How disappointing.

Does anyone else have any insight on this?