The 25-year-old's rehabilitation is progressing well with Fruean on track to return to contact training and playing early next year.
Chiefs coach Dave Rennie is happy with the final additions.
"Mils and Robbie are both proven talents and top men," he said.
"We know the value they will add to our side.
"Robbie is big and powerful and Mils is one of the most experienced players in New Zealand rugby so both are great additions to our squad."
Muliaina is 1.84m and 93kg. He has played 100 All Blacks tests and 104 Super Rugby matches for the Chiefs and Blues.
He won a Commonwealth gold medal at the Manchester games with the New Zealand Sevens team and played his ITM Cup for Waikato and Auckland.
Fruean is 1.90m and 110kg. He has played his Super Rugby for the Crusaders and Hurricanes and represented Canterbury and Wellington in the ITM Cup.
Muliaina will join the Chiefs in February at the conclusion of his Japanese season and Fruean will assemble with the Chiefs for pre-season at the end of November.