Victims of the regime had complained they cannot properly file as civil parties without knowing which sites are under investigation.
Questions over the handling of cases 003 and 004 have divided the Khmer Rouge tribunal (ECCC).
The court has completed one trial, of Khmer Rouge torture chief Duch, and is preparing for trial of senior leaders.
Rob Hamill's quest for justice is the subject of a new documentary, “Brother Number One”.
The international investigative judge for the court told VOA Khmer he anticipates at least two years of proceedings.
Members of the CPP have ignored a court summons for testimony from the former international investigative judge.
Thet Sambath said aspiring filmmakers who wish to do similar projects must be ready to work hard.
Some regime leaders have said in the past they had hoped to protect the country from Vietnamese occupation and others.
Victims from the Trapaing Thmar dam project say they want the judges to investigate closely Case 004.
Pen Sovann said the Khmer Rouge left him with “no choice” but to set up a movement to oppose them in the 1980s.
The extra hearing will be held just one month before the trial is scheduled to begin in September.
The UN-backed tribunal has come under increased pressure in recent months, after investigating judges concluded their work in Case 003.