The second season is completely written. Hugh Howey (the author of the series, and an EP on this series) said the second season was written before the strike, and they have already started filming it.
Actually, there are writers for this show. But they are covered under a different contract than regular TV writers, and aren't on strike. Almost all reality shows use writers who aren't covered by the striking WGA contract.
I don't think it will change much in the show, they can easily use the edit process to add the extra footage.
The one change we might see is more Reward Challenges. It was hard to have a RW and an Immunity Challenge in the same episode. Now we might see both more often.