It's currently not possible to do that!
As you can see in the following link, JRA-13245, there's already a Suggestion made about this matter.
You can vote on this issue to enforce your desire for this feature.
The only known way to do this is to "Convert Issue to Sub-task", but this action cannot be done in a bulk change.
Hope this helps!
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
We're bringing product updates and pro tips on teamwork to ten cities around the world.Save your spot