we don't have a dedicated importer/migration tool for Mingle. We're tracking such ideas under Epic: https://ecosystem.atlassian.net/browse/JIM-1169. Currently to migrate your data you'll need to use one of ours open-format importers such as CSV and JSON, you can find documentation for them here: https://confluence.atlassian.com/display/JIRA/Migrating+from+Other+Issue+Trackers.
Unfortunately I'm not aware what "export" features Mingle offers, but you should be able somehow dump it's data to a file and then change to it to CSV/JSON format which JIRA understands.
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