Unfortunately, the built-in building engine does not support the addition / generation of custom buildings.
You can add your own building using 3D markers.
But you need to generate your own building yourself.

I wanted to ask what is the best way to generate buildings from custom data using the OnlineMapsBuilding script. As far as I understand, the default script requests it's data from an Open Street Maps provider.

I have some custom lat/long data which make up the points of a building. I wanted to use these points as the points on the map which represent the flat polygonal shape of the building.  Is there a way of passing custom data (as a node) to the OnlineMapsBuilding script or do I need to write a custom script to do so?

EDIT

I think I need to be looking at the OnlineMapsBuildingBuiltIn.cs script. I guess I could adapt the code for the "Create" function and just pass in points directly without it needing to parse an OnlineMapsOSMNode?