Ability to map to more fields when creating issues using the CSV importer

With the new CSV Importer feature, non-admins can now create issues via CSV importing. However, users are only able to map fields defined in the Create Issue screen. Some circumstances require users to map to fields not present in the Create Issue screen. These are example situations

• Let's say that I have multiple PRs to be treated and whose correction is given to a subcontractor for development.
  Subcontractor does not have access to JIRA but I need to gather informations that were changed by the development team. Changes can be field modifications not present on "Create Issue" screen or status changes
  When starting development, project leader gives a csv export to the subcontractor, which in return gives a modified csv file that needs to be imported into JIRA by project leader.

• Let's say that we have a validation team, which cannot access to JIRA because they are working on the field or outside our premises. They may need to add comments, modify fields not present on "Create Issue" screen and even change status in a csv file that the project leader could import into JIRA

Under these circumstances, it would be useful if the CSV Importer have the ability to map to fields which are not defined in the Create Issue screen