Mr Cardoso accepted an invitation by former Spanish Prime Minister Felipe Gonzalez to join a team defending Leopoldo Lopez and Antonio Ledezma.
Mr Lopez and Mr Ledezma are prominent opposition politicians in Venezuela.
Mr Lopez has been charged with inciting violence and Mr Ledezma with taking part in an alleged coup.
Both deny the allegations. Their families have tried to enlist the support of influential international figures in order to secure their release.
Mr Lopez has been in detention for more than a year since he handed himself over to the authorities in February.
He is accused of inciting violence during mass protests he led in early 2014.
Forty-three people from both sides of the political divide died during several months of protests.
Mr Ledezma, a veteran opposition politician who was serving as mayor of Caracas, was arrested last month and charged with conspiracy.
Their supporters say the charges are politically motivated, but the government argues they want to violently overthrow the democratically elected government of Mr Maduro.
Mr Cardoso will join a team made up of Felipe Gonzalez, Peruvian ex-leader Alan Garcia, and former Canadian Justice Minister Irwin Cotler.
Mr Cardoso would travel to Venezuela to "examine the situation" of the prisoners, members of his centrist Brazilian Social Democracy Party (PSDB) said.
He is expected to be joined on the trip by Brazilian former presidential candidate Aecio Neves and Brazilian senator Aloysio Nunez.
The fate of the detained opposition leaders has divided opinion in Venezuela.
Their supporters say they are being targeted for their political views while the government accuses them of fomenting a coup.
Senior government officials regularly refer to Mr Lopez as the "monster of Ramo Verde" after the prison were he is held.
The announcement by Mr Gonzalez that he would be joining the defence team triggered a strong response from the Venezuelan government.
President Maduro accused him of forming part of an international conspiracy to overthrow the Venezuelan government.
Foreign Minister Delcy Rodriguez said Mr Gonzalez had "no right to act as a lawyer in Venezuela" and urged the socialist former leader to "find something else to do with his life and not act as a lobbyist for international and local right-wing interests".
Brazilian ex-President Fernando Henrique Cardoso will form part of a team defending jailed Venezuelan opposition leaders, his party says.