Zerkani is "the biggest recruiter of jihadists Belgium has ever known," Federal Prosecutor Bernard Michel said during a court hearing in February.
Zerkani was originally jailed in July 2015 to 12 years behind bars, even before the Islamic State attacks in Paris and Brussels that killed 162 people.
Zerkani, 42, was arrested by police in 2014 after he defied authorities for years with his off-the-radar recruitment of jihadists for Syria in the gritty Molenbeek district in Brussels.
"He's the emir, the leader...", a suspect identified as Yacine E. Told investigators in a deposition leaked to Belgian daily La Derniere Heure.