Hello, Thanks for explaining the problem.
For adding athermal material property using script is explaine here : Creating and modifying thermal materials from a script – Ansys Optics
More details are here: Materials in Ansys Lumerical HEAT — Lesson 1 - ANSYS Innovation Courses
You requirement is to add new material right?
If you duplicate a material in new name, you can modyfy its thermal property.
Yes you can modify the density, the specific heat, the thermal conductivity and the electrical conductivity.
Note: Any modification you make in the material database of your simulation file stays with the simulation file and won’t change the default material database.
Let me know how it goes and if further help required I will get back to you by Monday.
Thanks and regards.