Can a lower level Property add to an existing property or will it always overwrite it?

As far as I know, properties will override and not append when set at various levels. The only exception I'm aware of is system.categories. It does appear to append the various levels together (I believe it's internally stored as an array). I would be careful not to overload with manual categories though, it's heavy involved with controlling what checks are applied to devices.

Can you cover in more general what you are looking to do? Perhaps there are other ways to implement what you are looking to do.