You can easily create new JIRA issues (including sub-tasks) in GreenHopper, from the Planning Board, Task Board or Chart Board.
To create a new issue in GreenHopper,
Administrator's Tip: You can prevent issues from being created in a particular GreenHopper project by unchecking the 'Card Creation' option in the relevant project's settings. See Configuring your General Project Settings for more information.
Screenshot: Creating a new issue/sub-task in GreenHopper