In New Mexico the public service company, PNM, allows net metering AND purchases each kWh of power generated. It doesn’t seem that there is a way to model this with HOMER, is this correct?

I think what you mean is that PNM pays some non-zero amount for net excess generation.  You can indeed model that with HOMER.

For example, imagine that within a month you consume 500 kWh and generate 400 kWh.  Under net metering you would pay for 100 kWh that month.  But in another month if you consume 500 kWh and generate 650 kWh, your net excess generation that month is 150 kWh, and most utilities will pay you nothing for it.  In HOMER, under net metering the “sellback rate” applies to the net excess generation.  If the utility pays you nothing for net excess generation, you should set the sellback rate to zero.  If the utility pays you 2 cents/kWh for net excess generation, you should set the sellback rate to 2 cents/kWh.