Mas was only scheduled to stay at Les Bleus' training base in Marcoussis until the end of the week, but Laporte announced on Friday that the player would be staying on "definitively".
"Everything is going well for Sylvain," said Laporte.
"The only problem is we want to work on opposition scrums. He isn't yet ready for this so we brought Nicolas Mas in.
"It is better that he is with us for three weeks. At least now, he knows what we are doing and he is training here. I have therefore asked that he stays definitively."
Laporte added that Mas could even play a part in France's World Cup warm-up matches.
Les Bleus play England home and away and Wales in Cardiff before their first World Cup outing, against Argentina on September 7 - the opening day of the tournament.