In this article various methods of importing issues into GitHub are discussed. If you're looking for a solution to move GitHub issues between repositories check out this page.
There are several open source projects that use these APIs to facilitate importing:
Note: use these scripts and tools at your own risk.
Pro: there are hardly any limitations when using the GitHub API.
Con: using the API requires coding experience.
Zapier offers many integrations with GitHub:
Pro: Zapier offers a wealth of pre-made integrations powered by a point-and-click interface usable by non-coders.
Con: Most integrations are event-driven which means that every change in e.g. Trello is automatically synchronized with GitHub. If you want to cherry-pick issues then using Zapier is not ideal.
Releasewise connects to data sources like spreadsheets, CSV files, and applications like GitHub.
Importing issues into GitHub is simply a matter of dragging and dropping.
You even have the option to update the item states (e.g. "new" or "pending") of the source and destination issues while importing issues, and add comments to newly created issues.