Document List Properties in MS Office

Hi Group,
In SharePoint, we can define Custom required fields (columns) and use them f.i. to filter Libraries.

When I open an MS Office file where the required fields are not filled, I get a message that they are missing and I am required to fill them before I can proceed with the file.

I would like to be able to fill these properties with VBA, as I can derive them from the file name or contents.
Where are these properties stored in the file and with which object (read/write) can they be addressed in VBA? They are not (as I expected) included in the CustomDocumentProperties or DefaultDocumentProperties, although their lists are included in the file's XML.

Grtz, Bart