Lieberman has been briefed to oversee a shake-up of the agency's TV output.
His appointment is part of Campbell's ambition to change McCann's lacklustre creative reputation. Campbell took up his post five weeks ago.
Lieberman will work three days a week, but will leave once he and Campbell agree that the television department is in order.
A veteran TV producer, Lieberman was at AMV for more than 15 years and is credited with overhauling its TV department in the 90s.
He joined AMV in 1987, when the agency was known mainly for being a press advertising agency. Under his guardianship, it was crowned Most Successful Agency at the British Television Advertising Awards in 1996.
Campbell is hoping Lieberman will work similar wonders at McCann. "Lieberman is like the Godfather of TV production on the agency side," Campbell said.
Campbell was hired by Rupert Howell, who took on the job of chairman and chief executive last September.
Both Howell and Campbell are old friends of Lieberman. Campbell worked with him at AMV in the 90s, where he was a senior creative.