Tomkins, who played in England's Four Nations Series final defeat to Australia at Elland Road, has been granted permission to temporarily cross codes for the encounter.
Barbarians commercial director Mike Burton said: "We are delighted that Sam, with the support of the coaches and management at Wigan, has accepted this opportunity to play in such a high-profile union match.
"The Barbarian club has a tradition of thinking outside the circle when inviting players with league backgrounds, and usually to everyone's benefit.
"In recent years the likes of Mat Rogers, Lote Tuqiri, Wendell Sailor and, a few months ago, Willie Mason, have all made a mark when playing for us on the big stage. We are confident that Sam will be equal to the challenge this week."
The Barbarians, who will be coached by Graham Henry and Steve Hansen, have also added World Cup-winning winger Richard Kahui and Fijians Seru Rabeni and Nemia Kenatale to their squad for the encounter.
However, Wales fly-half Stephen Jones has been withdrawn from the squad as he is required by the Scarlets for their match against the Dragons on Friday.
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