Can we simulate the DC/DC Converter between Fuel cell and DC Bus with 90% efficiency?

HOMER's will not explicitly model the DC/DC converter, but you can account for the losses that occur in the converter by adjusting the fuel curve of the fuel cell.  Just divide the fuel curve slope and intercept by 0.9.  That way, the "fuel cell" component in HOMER will actually model the fuel cell plus the DC/DC converter.