Hi,
Well, just create a custom field of the table "product" via the menu Display>Custom fields and you will see it in the "custom fields" area of the product edition page.
If you don't, then there is a problem with the settings of your custom field. For example, you didn't select the correct table. In that case, please provide a screenshot of the settings if you don't find what's wrong and we should be able to tell you.
Once the field is created, you can use its column name in your CSV to import it.