Defending race winner Whincup was set to steer the car in the Pro-Am class alongside Tristan Vautier and Kenny Habul, but the team has confirmed it will now be run in the outright class with the addition of Raffaele Marciello.
“So there's the fourth driver, that will be him,” STM team boss Scott Taylor confirmed to Speedcafe.com.
“Under the rules of the 12 Hour we then become an all-Pro team; that's the holy grail, to be an outright winner in an all-Pro team.”
A former member of the Ferrari Driver Academy, and reserve driver for the Sauber F1 team, Marciello switched his focus to GT racing at the end of 2016.
The Italian joined AKKA ASP in the Blancpain Series, recording four podiums at the wheel of a Mercedes-AMG GT3 last season.
Whincup, the least experienced of the quartet of drivers in GT3 machinery, had his first outing in the car at Queensland Raceway yesterday.
The 2017 Virgin Australia Supercars Champion completed 40 laps as STM performed a shakedown of its newly rebuilt cars.
Team-mates Max Twigg and Craig Baird were also present, shaking down the team's sister car.
“They've both been completely re-built, so it was pleasing that the cars ran without fault,” said Taylor.
“We weren't chasing any issues or noises or anything, which is a credit to the boys who've done an amazing amount of work.
“We weren't chasing laptimes, we were more just chasing setup and things like that.”
The 2018 Bathurst 12 Hour gets underway at Mount Panorama on February 2-4.