How do I change the issue type from task to subtask after an import from a Microsoft Project file?
Currently, it's not possible to that from inside WBS Gantt-Chart, however you can do it in Jira and WBS will pick up that change next time you open it.
Here is how you do it:
Open the issue → select the action menu (…) at the top-right of the issue → click Convert to Subtask:

In the next screen, you will need to select the parent issue of the subtask and the subtask type:

Click Next, then Next again, and Finish after confirming the information on the screen:

And it's done! Next time you open your Gantt, that task will be a subtask with another issue as its parent.
You can't convert an issue to a subtask if the issue has subtasks of its own. You first need to move this subtasks to another parent.
For any other inquiries or issues, please reach out to us at our Ricksoft Support webpage. We will be happy to help you.