with the current (web-)interface, it’s IMHO not possible to select multiple tasks and move them as subtasks of another task. On the other hand, it’s easy to move multiple subtasks into tasks - so breaking it is easy
My suggestion would be:
If I select multiple tasks and drag them over to the “>” of another task, then the details of that other task should be shown. Then, it would be easy to drop the selected tasks as subtasks.
Thank you for considering (and implementing) this
For the asana (Android) app: While implementing subtask and multiple selection handling at all, this should be kept in mind.