Two men in custody over their connection to the murder of NSW police accountant Curtis Cheng will face additional charges when they appear in court.

Police on Thursday laid a further charge of knowingly participating in a criminal group against Talal Alameddine, 22, from Merrylands, who was previously charged with several gun supply offences.

Mustafa Dirani, from Marsfield, also 22, has had a charge relating to the supply of a pistol added.

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Dirani had previously been charged with knowingly participating in a criminal group, police said.

Mr Cheng was shot dead in October outside NSW police headquarters by 15-year-old schoolboy Farhad Jabar, who was himself shot dead by police.

Dirani was arrested in mid-November after being stopped in his car by Joint Counter Terrorism Team and Middle Eastern Organised Crime Squad officers.

Alameddine is accused of supplying a .38 Smith & Wesson revolver to Raban Alou on October 2 and it’s alleged Jabar used that revolver to murder Mr Cheng the same day.

Alameddine is due before Parramatta Local Court on February 4, while, Dirani is listed before the same court on January 14.