Will you be solving for the temperature of both the heat source and the heat sink? Typical thermal models would be 3D representations of the sink and source and the heat would be in the form of a flux, W/M^2, not just Watts. What's a physical description of what you are modeling? There won't be a simple way to load the temp-dependent heat source into Ansys. If the sink or source temperature was constant a temperature dependent convection could be defined between the two bodies.