Simple answer....You can't.
The bodies you see in your assembly you can consider as representations of your components geometry. There is no actual geometry present in your workpart. To be able to unite something you need geometry in your workpart.
You first need to create a wavelink or promotion to your toplevel of your assembly (your workpart).
The resulting bodies you can unite together.
There is one drawback and a logical consequence of this action...you will see two bodies..your components and your united bodies.
This can be solved by setting the reference set of your components to empty.