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.
The Documentation Center of Cambodia has distributed more than 400,000 copies to high schools across the country.
The tribunal has spent around $150 million since 2003, and began operation in 2006.
Court officials have denied any political influence in cases 003 and 004.
Former leaders Noun Chea, Khieu Samphan, Ieng Sary and Ieng Thirith have all denied atrocity charges.
Now 69, she suffers from knee problems, poor eyesight and insomnia, according to her lawyers.
The Center for Justice and Accountability is helping organize applications for victims of the regime who fled to the US.
Khieu Samphan acknowledged the Cambodian people had a pressing need to know what had happened.
Khieu Samphan also pleaded with the court to allow the witnesses submitted by his defense team.
A preliminary hearing this week has brought the amnesty deal made in 1996, to the forefront.