It is certainly possible to model a thermoelectric generator in HOMER, and it should be fairly straighforward.  The fact that the TE generator is either on or off has two effects.  First, you want to set the minimum load ratio to 100%.  Second, the fuel curve doesn't need to be accurate except at the rated output.  So I would set the intercept coefficient to zero and set the slope to whatever value gives the right efficiency.  For example, if the generator burns natural gas (LHV of 45 MJ/kg, density of 0.79 kg/m3) at an efficiency of 4%, the slope would be 1 /(0.04 * (45 MJ/kg / 3.6 MJ/kWh * 0.79 kg/m3)) = 2.532 m3/hr/kW.