Last month, Rajapaksa had named a three-member international advisory panel comprising Sir Desmond de Silva, Sir Geoffrey Nice and Professor David Crane to advice the disappearances commission headed by the former Sri Lankan judge Maxwell Paranagama.
The members of the panel Desmonde Silva and Nice are from Britain while David Crane is a US national.
Rajapaksa told reporters he may add another three advisers to make it six international experts.
Rajapaksa's nationalist allies, JHU and NFF have expressed strong opposition to the move. They have called it akin to "blindly walking into an international trap."
Also Read
"They will be there to advise not to investigate," the Lankan president said referring to the opposition.
He said the three international experts were appointed at the request of the Presidential Commission, which was set up in August 2013 and mandated to probe disappearances of persons between 1990 to May 2009 when the war with LTTE ended.
Sri Lanka faces an international probe over allegations that government forces killed about 40,000 Tamil civilians in the final months of fighting, a charge refuted by Colombo.
The nationalists warn that the three experts had served in various international war crimes tribunals from Sierra Leone to former Yugoslavia and their findings would spell trouble for the government troops.
Alongside the appointment of the experts, the mandate of the disappearances commission was also expanded to include if any person, group or institution had been responsible for any violations of international humanitarian law or international human rights law.
The UNHRC in late March had mandated the appointment of an international investigation team to probe Sri Lanka's rights accountability.