Pivac replaced Warren Gatland whose tenure ended with the completion of the Rugby World Cup. McNicholl played for Pivac's Llanelly Scarlets side while Halaholo plays for Cardiff Blues.
Wales first game under the new regime will be against the Barbarians in Cardiff on November 30.
Announcing his selection, Pivac told Welsh media, "I personally know a lot about Johnny and I had Willis in the Auckland age groups when I was coaching there, so I know his background.
"They're both exciting, attacking players. It's an opportunity for us to get Willis in for a week and have a look at him.
"We spoke to him earlier in the season about parts of his game we like and parts we think he can work on.
"It will be an interesting week to see how some players step up into a new environment and he's one of those players," Pivac said.
He and his coaching team were looking to evolve Wales' game with the ball over time.
He wanted to build on the strong defence system developed by Shaun Edwards who is now part of France's international management team.
"We've talked long and hard about how we're going to add value to what is already a successful rugby team. If something is not broken then we move on.
"We've been a very disciplined team, our record speaks for itself in terms of reducing penalties in games and opportunities for the opposition to get into the scoring zone.
"That takes pressure off the defence that has worked well which has been well documented over the last few seasons," Pivac said.
Those qualities had kept Wales in games and had resulted in wins. Many would say Wales were a hard team to break down and beat, he said.
"That's not a bad position to be starting from and we'll be looking to add value to the part of the game when we do have the ball.
"So the important part is that transition between attack and defence and how we go about that," he said.
Wales would not take a cavalier attacking attitude into games, rather they would look to evolve their game.
"It's going to be about managing expectations early on. I think a lot of people are going to expect us to be throwing the ball around willy nilly.
"I don't think that will be the case, it's got to be horses for courses," he said.