Mason, who has played rugby league for Australia, made six appearances for Hull KR before his failure to obtain a Tongan passport led to him being de-registered.
The 31-year-old forward has since agreed to join French club Toulon for 2011-12 and is on course to make his cross code bow at Twickenham.
Mason, who has experienced disciplinary issues throughout his career, played 24 times for Australia between 2002 and 2008 but has yet to make an appearance in union.
The earliest opportunity to feature will be against England, but the Barbarians also face Wales at the Millennium Stadium on Saturday week.
Alongside Mason, the latest additions to the squad include Edinburgh wing Tim Visser and Samoa international Iosefa Tekori.
The squad is comprised of 12 different nationalities with Georgia's Davit Kubriashvili and Erik Lund of Norway adding to the list.
Former New Zealand wing Doug Howlett and Clermont-Auvergne's Australian fly-half Brock James will join the squad in Cardiff for the second leg of the tour.
Italy stars Sergio Parisse and Leonardo Ghiraldini and France fly-half Frederic Michalak make their Barbarians debuts with David Young and Scott Johnson coaching the squad.
Barbarians squad: Forwards: S Bruno (Toulon), Q Geldenhuys (Aironi), L Ghiraldini (Benetton Treviso), C Hayman (Toulon), D Kubriashvili (Toulon), E Lund (Biarritz), S Parisse (Stade Francais), S Perugini (Aironi), G Smith (Toulon), I Tekori (Castres Olympique), I Thomas (Scarlets), P Tito (Cardiff Blues), J van Niekerk (Toulon), M Williams (Cardiff Blues).
Backs: B Baby (Clermont Auvergne), M Bastareaud (Stade Francais), D Howlett (Munster), B James (Clermont Auvergne), N Jeanjean (Brive), W Mason (Toulon), F Michalak (Toulouse), R Pienaar (Ulster), S Rabini (La Rochelle), P Sackey (Toulon), S Tillous-Borde (Castres Olympique), T Visser (Edinburgh).
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