The six now on death row were named as Ali Mojadam, Mohammad Reza Moghadam, Moein Khanfari, Habib Deris, Adnan Ghobishavi and Salem Mousavi.
Mizan Online said they had "carried out the orders of their European leaders such as Habib Nabgan and Habibullah (Habib) Chaab".
Chaab is an Iranian-Swedish dissident who vanished in Turkey during a 2020 visit and who is now on trial in Iran, which does not recognise dual nationality.
Iran has also accused Harakat al-Nidal of "cooperation with other terrorist groups" in a 2018 attack on a military parade in Ahvaz that authorities said killed 25 and wounded almost 250.
The Islamic Republic of Iran executes more people every year than any other nation except China, according to rights groups including Amnesty International.
Last month, Iran's judiciary put to death a Kurdish militant who had been convicted of killing a police officer, a move condemned by rights groups who called him a political prisoner.