Mamoepa said that besides Mugabe and Tsvangirai, Mbeki will also meet the leader of a smaller MDC faction, Arthur Mutambara.
It was unclear, however, whether the meetings would be held separately or in one session.
Edwin Mushoriwa, a spokesman for Mutambara, told the AFP news agency that all three Zimbabwean leaders would meet soon in Harare.
Mushoriwa said negotiators were expected to return home from South Africa on Friday and the meeting would take place afterwards.
Business Day, a South African newspaper, reported that Mugabe and Tsvangirai will meet on Sunday as the two sides draw nearer to a power-sharing agreement.
|Mutambara is among three Mugabe opponents expected to hold a meeting soon [AFP]
Mbeki's trip comes in advance of Zimbabwe's Heroes'Day on Monday, which commemorates fighters who died in the guerrilla war that led to independence in 1980.
Mugabe himself was seen as a liberation hero in the war and was propelled into power afterwards, where he has remained ever since.
The 84-year-old was re-elected in a June 27 presidential run-off that was widely condemned as a sham after Tsvangirai dropped out of the race, citing violence against his supporters that had killed dozens and injured thousands.
Tsvangirai finished ahead of Mugabe in the March first round of the election, and the two sides entered power-sharing talks following a July 21 deal laying the framework for negotiations.
In a sign the two sides were moving closer to an agreement, the parties issued a joint statement earlier this week calling on their supporters to halt political violence.
The main sticking points in the talks are believed to involve what roles Mugabe and Tsvangirai would play in a power-sharing government.