Create subtask under existing subtask

Problem Definition

Currently in an Issue, user can create sub-task but user cannot create sub-task from existing sub-task.

Suggested Solution

User would like ability to create sub-task under a sub-task. Basically the sub-task is a child sub-task under existing sub-task.
User would like sub-task structured as shown in the screenshot in attachment.

Workaround

There are no workaround for nested sub-task function.