Johnstone's Super 14 season was ended this weekend by an operation on a finger on the recent South African tour.
"There was some irony in that if the operation was successful, he would be ruled out for the rest of the campaign," coach Robbie Deans told The Christchurch Press.
"But if it had not gone as well as hoped, he could have played for us again later."
Australian-born Franks, 21, joins brother Ben - also a prop - in the squad.
Otherwise, Deans should have a full squad to pick from after flanker Kieran Read recovered from a shin injury picked up in Friday's 34-7 win over New South Wales Waratahs.