Nope...Joseph was not the Foster Father...He was technically the Step-Father...The husband of Mary...Thus the real King of heaven...If Mary is the Queen, Joseph is the King...
As for Joseph's relation to the Royal Line of David, he was of the House of David in his own right. If you want to explore all the dynastic or courtly implications thereof, go right ahead.
But it does not automatically follow that if he was the husband of Mary and step-father of Jesus, he was King of the Universe. It doesn't work that way, for instance in Jewish monarchies. Was Jesse, as David's father, considered King of Israel? You see the problem.