You'd need to write a new custom field type to be able to do that. The logical structure of the internal cascading select is parent/child, not free-form.
You might be able to do it with two standard select lists though, linked by Behaviours from the Script Runner add-on
If you spend enough time as a Jira admin - whether you are managing a single, mid-sized instance, a large enterprise one or juggling multiple instances at once - you will eventually find yourself in ...
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