Hi Loris! Thanks for your question
Could you please explain the purpose of the mesh you want to add? Is it placed in the outer surface or the inner surface?
I guess it's in the outer surface and the goal is to act like fins, increasing the heat exchanged (with the ambient I guess?)
In that case, you could achieve it in different ways: you could build a more detailed thermal network for the regenerator, add a Convective Heat Transfer block between the mass and the ambient (represented by an ambient temperature source) like in the picture below:
Or you could customize the Pipe (G) block to create your own pipe with a detailed modeling of heat exchange with a mesh.
In any case, I definitely NOT recommend to use a Simulink subsystem to represent a physical device in your Simscape system, because it can lead to performance issues as they are being solved simultaneously but using different solvers that might compete between each other for precision and resources. Simulink should be use for the control algorithms of your systems, after having interfaced with sensors and actuators.
Hope it helps!