This was mentioned here, 10 years ago: How do I move a task to a subtask Asana via the API - Stack Overflow
A software engineer at Asana said “We hope to allow that in the API soon”.
Is this available yet?
I would like to have a task that exists as a task in one project and as a subtask to a task in another project. This is possible in the frontend. Albiet, first you need to convert the task to a subtask, then re-add it to the project it was in previously as well.