Surely you can't be outside the playing area when you push the ball down? Do you mean you can exit the field of play, re-enter and THEN push the ball down?
Law 8. Scoring
2: A try is scored when an attacking player
e. who is in touch or touch-in-goal, grounds the ball in the opponents in-goal provided the player is not holding the ball.
So, you can be out but the ball is still in, and ground the ball as long as you dont pick it up.