Sub project inside activity?

Hi,

I'm starting with ProjeQtOr and I'm trying to get the right structure for my projects and activities. I've created a project for a product, which consists of multiple PCB's. For each PCB I've also created a project, to allow me to use a project template to instantiate the correct sub-structure.
The product project has various phases (activities) of which one is 'Prototype'. As part of that phase a PCB gets developed, so I want the PCB project to be inside or below that phase. Unfortunately it seems to be not possible to make an activity of the PCB project a sub activity of the Prototype activity. Nor is it possible to make the whole PCB project a sub element of the Prototype activity.
I've attached a screenshot where I've made the first activity of the PCB project a succesor of the Prototype activity, as that's the closest I could get, but I would like the Prototype activity to become a parent activity.
What would be the best way to model such a relationship?