Ravalomanana's grouping and that of Albert Zafy, another former president, inked the deal, brokered by the Southern African Development Community (SADC) in an attempt to resolve the island’s prolonged crisis.
Eight other political groups had already signed.
But the political movement of ex-president Didier Ratsiraka has still not done so.
Agreement was held up in June when an SADC June summit called for the
unconditional return of Ravalomanana, who is in exile in South Africa, and current ruler Andry Rajoelina rejected the condition.
Rajoelina, who seized power in March 2009 after an army-backed coup, finally agreed to Ravalomanana's return following lengthy negotiations.
But Ravalomanana may choose not to go back as he faces life in prison back home after being sentenced in absentia for the 2009 killing of protesters by his presidential guard during violent street demonstrations that led to his overthrow.
Mamy Rakotoarivelo, Ravalomanana's representative at the Saturday signing said the former president could "return as soon as new transitional institutions are set up."
The agreement also calls for a power-sharing arrangement by the signatories until presidential and parliamentary elections are held, with Rajoelina in the interim remaining the president.