The National Human Rights Commission announced a probe into the high-profile case last month, after the top UN human rights official on North Korea met some of the women and urged an investigation.
The issue has long been controversial, with Pyongyang saying the women were kidnapped from a North Korean state-run restaurant in China while Seoul insists they defected of their own free will.
"We have interviewed some of the waitresses," an official at the commission said on condition of anonymity.
"We plan to meet the others as well if they are willing to talk to us," he said, declining to give details.
The probe follows a call by Tomas Ojea Quintana, the UN's Special Rapporteur on human rights in the North, who asked Seoul to carry out a "thorough and independent investigation" and to "hold to account those who are responsible."
In a bombshell revelation in May, the manager of the restaurant where the waitresses worked said that he had lied to them about their final destination and blackmailed them into following him to the South.
Heo Gang-il told the South's JTBC television that he had been recruited by Seoul's National Intelligence Service (NIS) in 2014.
Fearing exposure in 2016, he asked his NIS handler to arrange his own defection. But at the last minute his minder threatened to report him to the North Koreans unless he brought the waitresses with him.
Seoul's Unification Ministry has previously said the waitresses defected to the South of their own free will.
Pyongyang has repeatedly insisted the women were abducted, while rights activists in the South have demanded an investigation.