Another way is to upload the sub-tasks through a CSV file. As long as you reference the parent issue ID (or KEY) you can put in at least issue type, summary, description and parent id/key. JIRA will populate the field(s) for Reporter, and key for the new issue. I'd try it with a few issues and in a test instance or project beforehand to insure you understand how to do CSV imports.
Welcome to the community, @Raphael Kwawu.
Your use case is possible with our app Deep Clone for Jira.
You simply have to:
Doing that you can Bulk Clone the child issues and update the Epic Link in one step.
Don't hesitate to contact us if you have questions or feedback.
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