Currently there is no API for Analytical or Schematic Constructions. Having an API and managing Constructions as types in Revit would be the optimal, I will bring it up to the team. We have other similar requests for this.
Adding to or changing the system file Constructions.xml is not a good idea. It might work for the moment, but you can lose your construction data when you upgrade to a newer version of Revit.
You can specify analytical thermal properties on the building elements (like Walls, Roofs etc) and they can be transformed to construction data in analysis or calculations. Can that be a solution for now? It depends a bit on the use case there?
Fredrik, do you use Export to gbXML, Energy Analysis, Heating and Cooling Loads?
Fredrik, I would also recommend searching for a corresponding wish list for this in the Revit Idea Station, and adding your comments there, or creating a new entry, if there is none already present there yet for the suggested functionality:
https://forums.autodesk.com/t5/revit-ideas/idb-p/302
Tag it as an API wish:
https://forums.autodesk.com/t5/revit-ideas/idb-p/302/tab/most-recent/label-name/api
Ensure it gets as many votes as possible to underline its importance to you and the rest of the developer community.
The Revit Idea Station is currently one of the main driving input forces for Revit API enhancements.
The Revit development team look there. Your comment here in the discussion forum might be overlooked.
Thank you!