"We push to merge with all parties, politicians and civic groups that uphold the Constitution and the principles of the free democracy," the party was quoted as saying by the Yonhap news agency.
The launch of the new Party for Future Integration is expected as soon as Sunday. It will have 115 lawmakers in the 295-seat parliament, where the ruling liberal Democratic Party has 129 seats.
South Koreans will go to the polls on 15 April to elect the national legislature under a mixed electoral system. The main opposition party reportedly plans to create a satellite party in a bid to snap up seats allocated through proportional representation voting.