My question is if it is possible to implement the environmental energy act of the german government in the simulation of a on grid system?  In Germany, the power companies have to buy kwh from the producers of renewable energy for a certain price per kwh.

I know of at least two variants of such feed-in tariffs. 

Say that the power price is $0.10/kWh and the feed-in tariff (what the utility must pay you for the renewable energy that you produce) is $0.40/kWh.  And say that in a given month you produce 1000 kWh and consume 650 kWh.

I think most commonly, the utility company will pay you for the excess energy that you produce above and beyond your electrical demand, which in our example is 350 kWh.  If this were the case, you would receive $0.40/kWh

* 350 kWh = $140 for that month.

But in some jurisdictions, utilities must pay the feed-in tariff for all renewable energy generated.  If that were the case, you would buy 650 kWh at $0.10/kWh, and sell all 1000 kWh at $0.40/kWh, so you would receive $0.40/kWh * 1000 kWh - $0.10/kWh * 350 kWh = $365 for that month.  So the second variant is more generous. 

To model the first variant in HOMER is simple.  Choose 'net metering' and set the power price to $0.10/kWh and the sellback rate to $0.40/kWh.  But the second variant is effectively two separate systems as far as HOMER is concerned.  So you can model a renewable power source connected to the grid with a sellback rate of $0.40/kWh, and in a separate .hmr file model a house that buys grid power at $0.10/kWh.  You may choose to not even model the house and electric load, and instead just model the renewable power system.