Can you add extra columns to a fixed choice list field?

No. A fixed choice list is a field that contains a list of static values populated from an FND lookup type. The fixed choice list displays the lookup type's values in a single column, and you can't add more columns to this field.

You can, however, make the values in a fixed choice list more descriptive, which could help users more easily make a selection at runtime. To do this, access the lookup type in the Setup and Maintenance work area and modify the meanings for each value in the lookup type.

Or, you can instead create a dynamic choice list field in Application Composer. Dynamic choice list fields can display multiple columns in the list of values at runtime.