Yes, you can even put them in up to the 54th trait
You can then put more points into the weapon, but the opinions on this vary: I vaguely remember that there was an official statement that Blizzard wants to actively limit people from already farming now for the 7.2 traits and will come up with a way to discount points collected before 7.2, but can't find that statement anywhere any more.
I personally am now working on my 2nd/3rd weapon because I simply cannot believe that Blizz would allow people to already grind enough power to get to the 7.2 traits.