Hirakawa won last year's Le Mans with New Zealand's Brendon Hartley and Switzerland's Sebastien Buemi as well as the endurance world championship with Toyota, a series the trio currently lead in the Hypercar category.
McLaren said he would support them in the simulator as well as track testing while continuing to race for Toyota in world endurance.
"We look forward to working closely with Ryo who has already spent time on the simulator...his wealth of experience will benefit the team in a number of areas as he joins our strong stable of drivers," said McLaren team boss Andrea Stella.
McLaren, whose regular race drivers are Britain's Lando Norris and Australian Oscar Piastri, also have access to Mercedes F1 reserve Mick Schumacher this season.
The former champions had double IndyCar champion Alex Palou as another reserve but that deal ended when the Spaniard's planned move to McLaren's IndyCar team fell through.
