Halaholo qualified on residency grounds last month having been at the Cardiff Blues since 2016 while McNicholl, who plays for Scarlets, became available for Wales on November 3 after completing the controversial three-year conversion period.
The pair, both 29, will be some of the last players to sneak in under this criterion – often dubbed the project player rule – before it is extended to five-years from 2020.
McNicholl and Halaholo are among five uncapped players joining 22 members of the Welsh World Cup squad who finished fourth in lining up against the Barbarians next weekend.
"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," Pivac said. "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.
"It is great to get the squad announced and get the ball rolling on our first meet-up and first fixture next week.
"This Barbarians game is a great chance for us as a new squad and management to get together and to set the scene for what we are looking to do. Having this opportunity ahead of tournament rugby in the Six Nations is ideal for us.
"It is also a great chance for some players to really put their hand up and show what they are about and take to the field in front of a big crowd at home."
