As of now, you cannot add table items in NXT for custom fields. e.g., if I'm in a donor record and I have a custom field for "favorite color", it does not give me the option to add another color to the table. I can only select what's already available in the table. This functionality is available in database view. Please make it available to some capacity for NXT, as sometimes table entries need to be revised with changing priorities or for data cleanup.