The court found that Soughir and his small Belgium-based group, which included his younger brother, Souhaieb, helped recruit and send fighters to Iraq, including Muriel Degauque, 38, who died in an attempt to attack a US military convoy.
Her explosives went off prematurely during the incident on November 9, 2005, no one else was killed in the explosion.
Degauque, the daughter of a hospital secretary, grew up in a town outside the city of Charleroi before becoming a baker's assistant.
She became involved in Islam after marrying an Algerian. Belgian prosecutors say she entered Iraq from Syria a month before her attack.
Authorities say the Belgian group Degauque became a part of had embraced al-Qaeda's ideology.
The group included her second husband, a Belgian of Moroccan origin, who entered Iraq with Degauque and was killed while allegedly trying to set up a separate suicide bombing.
Authorities said the Belgian network had been planning to send more volunteers to Iraq for attacks.
Defence lawyers for the six argued they were not terrorists but rather freedom fighters opposed to US forces in Iraq.
The reading of the 200-page verdict took most of the day at the city's palace of justice, which was under tight security due to recent threats in the Belgian capital.