Unless I'm mistaken, what you're trying to do is not possible.
In the "normal" mode, part bodies can only contain 3D functions. So everything else has to be somewhere else and is stored in a geometric set.
In hybrid mode, bodies can contain all types of objects. But suddenly there is no need for a geometric set.
On the other hand, I think it is possible to reorder your tree to have a succession of body/geometric set/body/... with the elements in the "good orbre".
You can't move a geometric set in a body of pieces in hybrid mode.
You have to use the Geometric Ordered Sets .
In normal mode, you can move the geometric set into a part body (right click on the set / Object set... / Change geometric set) and then point to the part body or you want to integrate the set.
As Chamade says, in hybrid mode, we integrate construction elements, plane stitches, etc. directly under the body of the parts. the objective being to have a chronological reading of the tree.