I don't think that's possible. I think you have to pay the $5 Airtrain fee to get across the fence from airport property into the Howard Beach subway station, even if you didn't take Airtrain.
You can, however, take a transit bus to a different subway station, and you get a free transfer to the subway if you pay with a Metrocard (probably available at newsstands in the airport).
I don't think that's