yes, you can print sub-tasks as well.
The best way to build a custom InstaPrinta template, is to customize an existing template. Just download the template in the global InstaPrinta configuration. You will get an XSL file, which you can customize to your own needs.
If you want to change the JIRA fields on the template, just change the InstaPrinta placeholder (e.g. ___ip.assignee___).
You'll find additional information about XSL-FO at the end of the InstaPrinta Administrator Guide.
If you have further questions or need support for creating templates, feel free to ask us.