Utility back Steyn and second row Muller have not featured as much as they would have liked for the world champions this month but both could well be facing the Lions next summer.
The pair join fellow Boks Bryan Habana, Jean de Villiers, Fourie du Preez, John Smit, Bakkies Botha and Schalk Burger, as well as former South African international Percy Montgomery, in the Barbarians line up.
Steyn replaces injured French star Francois Trinh-Duc while Muller comes in for England skipper Steve Borthwick.
Front rowers Federico Pucciariello and Census Johnston have also been called up in place of Gurthro Steenkamp and Carl Hayman respectively.
The Babaas will announce their starting XV at 12pm this Monday, December 1.
Backs: Percy Montgomery (SA), Bryan Habana (SA), Joe Rococoko (NZ), Shane Williams (Wales), Rico Gear (NZ), Jean de Villiers (SA), Ollie Smith (England), Francois Steyn (SA), George Gregan (Australia), Fourie du Preez (SA)
Forwards: John Smit (SA), Mark Regan (England), Census Johnston (Samoa), Federico Pucciariello (Italy), Patrice Collazo (France), Bakkies Botha (SA), Johann Muller (SA), Chris Jack (NZ), Nick Koster (Western Province), Schalk Burger (SA), Jerry Collins (NZ), Richie McCaw (NZ)