In More Actions tab, instead of create sub-task, we need customized option, say Expand user story.

Rohan Kulkarni1

After creating an issue, when we click on More Actions, we need a customized option instead of create sub-task. Either we can replace create sub-task option by expand user story(customized option) or we can add a new option Expand user story along with create sub-task.

RambanamP

you can add your own options check the following doc

https://developer.atlassian.com/display/JIRADEV/Plugin+Tutorial+-+Adding+your+own+Menu+Items+to+JIRA