To create a new vertex group, go to Data tab, click + in Vertex Groups, double click on the Group name and rename it to bonnet.
Process of creating a vertex group
Change Object Mode to Edit Mode.
Switch to Face Select.
Select all faces by pressing A or using the CTRL+A shortcut.
Go to Vertex Groups, click Assign button and exit Edit Mode.
Now all of the selected faces are properly set to the bonnet vertex group.
Assignment of faces to vertex group
Hierarchy
Our new custom bonnet model has to be considered by Sollumz as a valid part, so we have to move the mesh by expanding Adder's armature then drag and drop bonnet's Drawable Model inside adder.mesh
Bonnet part moved into adder.mesh
You can delete the original bonnet, since is not needed anymore.
Removal of vanilla bonnet part of adder.
Armature
A replacement custom part has also to be linked to the armature, you can do this via Armature modifier.