Hmmm. This is not in ‘Boards’ view, is it? At least I can’t figure out how to select the Destination Task AND keep it visible while I then selected the Eventual Subtask to be dragged and dropped.
Unfortunately, this would need to be done in a list view. The good news is that you’re not stuck…
You will need to do an advanced search, targeting only that board project. This brings the tasks up in a list view. Yay! Now you can highlight them all and either assign them to yourself to edit in my tasks OR add (multi-home) them to a blank temporary list project. Then you can do the above method (assuming the tasks and subtasks are on the same board).
Note: Thanks to @Kevin_Leggett for testing this, but you can NOT drag tasks in advanced search mode so you MUST add it to another project or your task list.
If the tasks aren’t on the same board or you don’t have Advanced Search capabilities (paid version) do the following:
Create a new list project to temporarily hold tasks. Add (multi-home) the board tasks that you need one at a time to a temp list project as you can’t multi-select tasks on a board. Now the tasks are in two projects. At this point you can drag items into the tasks as noted in previous posts from the temp list project. Once finished you can simply remove those tasks from the temp list project (keeping them in the board project) and now you have Board tasks with the subtasks you needed. Remember you can select multiple tasks in the list view to remove them all at once from the temp list project.
Not ideal… but it should work.
As a side note, this is why I ONLY use board tasks as project overview boards where the board tasks are multihomed “information” tasks for various other projects that I move between “stages” (columns) on the board project but allow me to easily see custom fields for both projects and click into the parent project. Boards are good for very specific use cases… if you want to use boards for everything you do… just use Trello. The power of Asana is in the list view. They could add features of the list to the boards at some point, but the nature of boards make it hard to do what you can do in list view… that’s not Asana’s fault but merely a logistics/UI/UX problem inherent with boards and why we left Trello…
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I was unable to drag and drop the tasks under the advanced search/saved search report results… however was able to use the ‘my tasks’ sorted by project (after assigning tasks i wanted to move to myself) to get the same result (open the target task - appears in right pane, then i was able to drag other tasks as subtasks).
Hi Matthew. Thanks so much for this detailed and hugely helpful response. I’ll try what you suggest, but take my time at it!
You’re correct, you can’t drag from advanced search. 
Your solution works if none of the tasks were already assigned to you. I guess you could sort them by project in your my tasks and go that route. But rather than junk up your My Tasks, its nearly as simple to just make a blank project and multi-home them to that.
The beauty of the advanced search is that you can multi-select tasks that are stuck on boards.
I’ve updated my instructions above. Thanks for the feedback!
Thanks Matthew, I agree blank project is the way to go. It was getting messy pretty quickly so I tried the blank project which was much easier. I hadn’t spread tasks across multiple projects before and now I suspect I will do it all the time, especially with my board projects.
Hello! To make one task a subtask for another task, I need to drag it to this task. In this case, both tasks should be in the same project. Sometimes tasks can be on different projects.
It would be great if it was possible to assign a parent task to the task. For example, through the menu item Set parent task.
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Hi @elenka and thanks for the feedback! Just merging your post to Convert task to subtask to avoid to many duplicates! 
Not sure if this idea has been added before? But it would be super useful if I could just use the @mention feature when creating a subtask to make the linking of tasks as sub tasks 100x quicker. I’ve mocked up a screen grab of the idea!
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I have been using this in Wrike. Super useful
I’ve just tried this and one thing that i fine anoying is that when i drag my task become subtask it loses all its settings - Projects, Custom fields, etc.
I would like to keep these.
It actually doesn’t “lose” them. They just become inaccessible. If you add information into the custom fields then move the task to sub task and click into the subtask the custom fields will be there but grayed out. Also you can have subtasks be part of other projects. Just hit Tab+P.
If you move it back to being a regular task you can edit the custom fields then move back to subtask and it retains the information. This is similar to adding custom fields from multiple boards on one task then removing it from a board. It retains the information but is grayed out till you add it back to the board containing the custom fields.
While this isn’t ideal… it works.
Having said that, this is just one more reason NOT to use subtasks. They hide information and are not conducive to getting work done.
Hi Matthew,
Thank you for the response. I get this and dragging to convert tasks to subtasks and vice versa is quite simple.
I agree that subtasks are hiding information, but that is the reason i would like them innitial project settings when converted from task. This way they would appear in both/multiple projects.
My use case is following.
I have project called “Company goals” and bunch of other projects.
If i create “New task” with certain custom fields under project X and drag that task to become subtask of one of my tasks in “Company goals” project, i would expect “New task” to have both projects set up, to keep custom fields and when displayed in Project X view it would show custom fields and both projects set up.
Currently this is not happening.
Does this make sense and do you have some other workaround for this workflow?
Umm I’m not sure I fully understand, but I think a solution would be to drag it over to be a subtask and then hit Tab+P shortcut to add it back to the old project. In this way a subtask can also be a regular task of another project
Thanks to all of you on the thread - v helpful. I wanted to nest my silly number of tasks under fewer projects and ‘lead’ tasks, using subtasks. Opening a new task and then dragging a task into the subtask area worked beautifully (initially I had to open a blank sub-task to show the area). Thx R
perfect - this worked for me
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I’m looking to multi select some tasks to drag inside of another task to make them all subtasks. It looks like this isn’t currently possible?
I see that a task can be subtask of only one task and not multiple ones.
When I drag one below a second parent task, it disappears from first one.
Is there a way to have them below multiple tasks?
Thanks,
Francesco
P.S. Anyway process of converting tasks to subtask is not so elegant and quick