Koroma's office issued a statement saying the former president would honor the invitation.
Zimbabwe's main opposition party CCC's spokesperson said the party has appealed the ruling at supreme court.
The conflict has uprooted some 630,000 from their homes.
Leaders at the event rallied on Africans to decolonise their minds to thrive.
The G5 Sahel was created in 2014, with an anti-militia force added in 2017 backed by France.