Dear Maxime, welcome to the forum. I am not a Dynamo expert, so I can only address the pure Revit API side of things:
Q: generate toposurface:
- The Building Coder: Import LandXML Surface
- The Building Coder: Creating Topography Contours and Building Masses
A quote from the latter: “Developing this application is only possible in Revit 2014 (and later) in which the API library includes constructors for SiteSubRegion and BuildingPadType.”
Q: create building pad:
Yes:
Q: create subregion
Yes:
Q: create masking element?
This I don’t know, cf.:
I would suggest asking and discussing this last question in the Revit API discussion forum, continuing that thread.
Cheers,
Jeremy