I'm looking to accomplish this feat with odoo(v9):

-service product

-invoicing policy: Ordered quantitites

-track service: Create a task and track hours

In this way, when a salesman sells this product (eg. a prepaid packet of hours), a new project (linked to the sales order) with a new task(with the total amount of hours) is created. This is very nice.

Sometimes though is hard to use the timesheet lines in this task to track activities, it would be easier if one could create some kind of "child-tasks" or "linked-tasks" which can follow a normal scrum flow(with stages) inside the same project, and the hours tracked inside them are going to be subtracted from the parent task. 

Is this possible?