Do I understand you right that you want to have a discontinuity in your thermal resistance of your model. If yes, then i can refer you to Subdomains and boundary conditions
This should be applicable for your example.
Note that the c+±Code at the bottom is not quite up to date; you have to add some header files and stuff below the code to make it work (see Pass Function to C++ expression - #8 by Emanuel_Gebauer) 