If NetVolume of Wall components has to be written in a custom pSet (if this is a special requirement), this Network maybe provide a solution.
If you export Base Quantities, you get component volumes exported. Which is the normal-case to export the component quantities.
What does the Script(s) do:
1. The first Script creates record fields for every component Name exists in the Document (Walls have to be in the drawing, not just as a style).
2. The Second Script gets Wall component NetVolumes and writes it in the Record attached to the Wall for every component Name.
3. So you are able to use the Data-Manager to attach this component informations to the Wall Body and be able to export is as a Custom pSet to ifc (if this is a requirement in a specific project)