Track Sub-Task Time without assigning parent task

[r.engel]

Me again with another question. 

Situation:

- Parent Task --> not assigned OR assigned to User 1

- Sub-Tasks --> assigned to other Users, like User 2,3,4

Objective:

I want the times of users 2,3,4 to appear in the sprint planning in the backlog and the above "estimated  time per user" overview. 

For now this does only work if the Parent Task is assigned to the same user as the Sub-Task.

Can anyone help?

Thanks

Rafael