Update existing JIRA issues using CSV import

I'm trying to import updates to existing JIRA issues but I'm not able to find 'Issue Key' as a field to map to when setting up the configuration file. If I try to import a file that has the issue key populated with an existing id, it still creates a new issue. There are a few other fields in my import file that aren't appearing in the configuration mapping; custom 'security', status and updated. Any ideas what I'm missing?