You need to select both parts. To do this, you can click on the arm, then hold [Ctrl] and click on the hand. The order is important to make the pivot point at the shoulder. I find it easier to just select the entire arm in the Graph tab. Click on the arrow next to the group name of the upper you are working with. This will open the group and now you can select the Right or Left groups. These include the arm and the hand. If you toggle the Lock icon on for the Right or Left group, then any time you click on the part in the main window, you'll select both the arm and hand, with the correct pivot point active.

I've yet to find a way to easily rotate the hand though, as its pivot point is in the grip area of the hand rather than the wrist so it doesn't stay inside the arm if you rotate it on a single axis 😦

If you hover over the hand, you'll see two dots, one at the grip point and one on the wrist. Click on the dot at the wrist and your Gizmo will jump there. Now you can rotate it on the wrist.

Oh, thanks! Never noticed that before. Do you know if that pivot point is included somewhere in exported Blender files?

I'm not sure. If it is, I don't know how to switch to it. Sorry.

No it is not exported to Blender. I created a special abstraction layer for Mecabricks in order to handle these points.