At London's Old Bailey court, the British-Romanian dual national admitted an offence of conspiracy to assist unlawful immigration.
He is still charged with 39 counts of manslaughter, along with Eamonn Harrison, 23, from Northern Ireland.
Two other men, Ronan Hughes (41, from Ireland) and Maurice Robinson (26, from Northern Ireland), are also accused of immigration offences.
Robinson, who drove the truck, pleaded guilty to 39 counts of manslaughter at a trial in April. He had previously admitted conspiracy to assist unlawful immigration.
Meanwhile, Hughes pleaded guilty to manslaughter on August 28.
On October 23 last year, bodies of the 39 Vietnamese victims were discovered in the back of a refrigerated lorry at an industrial estate in Grays in Essex, near London.
Autopsies concluded that the provisional cause of death of the victims was a combination of hypoxia, or oxygen deprivation, and hyperthermia, or overheating, in an enclosed space.