They are charged with atrocity crimes, including genocide, with a trial expected later this year.
Tribunal spokesman Lars Olsen said the Trial Chamber will consider the request as planning for the trial moves ahead.
Rapp, who staid in Cambodia Thursday through Sunday, said his visit had convinced him the investigations will go forward.
Four seniors leaders—Nuon Chea, Khieu Samphan, Ieng Sary and Ieng Thirith—are awaiting trial for atrocity crimes, including genocide.
Around 2,120 victims who have registered as civil party complainants have been accepted from more than 4,000 applications.
Under the Pol Pot regime, people were taught to keep silent, and often their survival depended on that silence.
Professor Sum Chhum Bun says the literature, arts and culture during the regime were used for Marxist-Leninist ideology.
The Khmer Rouge tribunal sentenced Duch to a commuted 19 years in prison, earlier this year.
More than 8,000 victims have filed with the unit and are awaiting the trial of four jailed senior Khmer Rouge leaders.
In a December report, OSJI says donors can push harder for wider indictments and great participation by leaders.
Siegried Blunk, of Germany, will replace Marcel Lemond, who resigned in September.