HiHi and thanks for the question. There is no law that prevents a player from voluntarily going to ground with the ball. However once he has done so he is out of the game and must immediately do one of three things.
Is a player going forward with the ball allowed to go to the ground to avoid being tackled by an opponent?
The game is played only by players who are on their feet.
1. Players, who go to ground to gather the ball or who go to ground with the ball, must immediately:
a. Get up with the ball; or
b. Play (but not kick) the ball; or
c. Release the ball.
2. Once the ball is played or released, players on the ground must immediately either move away from the ball or get up.
3. A player on the ground without the ball is out of the game and must:
a. Allow opponents who are not on the ground to play or gain possession of the ball.
b. Not play the ball.
c. Not tackle or attempt to tackle an opponent.
The Rugby Ref