As described in the two documents below, the External System Import functionality is different then the Create issues from CSV.

• Creating issues using the CSV importer
• Importing data from CSV

However, the statements below lead to believe that it is possible to map both the reporter and the assignee when performing a Bulk Create from CSV operation:

If you are importing a username-based CSV field (e.g. Reporter or Assignee) and you do not check the Map field value option for this field, then the import process will automatically map imported usernames from the CSV file to (lowercase) Jira usernames.

You can choose to have the importer automatically create Jira users for any values of the Assignee or Reporter field.

The problem is, whenever trying to map the Assignee field, it is not returned in the existing Jira fields:

Steps to reproduce:

1. Click Issues > Search issues
2. Click the ... button and select Import issues from CSV
3. Upload the CSV file and try to map the Assignee field