In the Configuration Options, set the following:

1. Allow Additional Choices For: Items (specifies whether additional item choices can be selected).
2. Additional Choices Filter (optional; specifies the filter to apply to the additional item choices that can be selected from.

See the following table for an example configuration:

Once the Additional Choices configuration is completed in the Option Choices, click Select from Additional Choices to open the item (or catalog item) list and select an available item (or catalog item) to add to the configured item. When the user selects an item as an additional choice, the item is added to the Option Choices if it is not already added to the configured item. If using catalog items, it creates the item before it is added to the Option Choices.