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 and then joining wealthy French club Toulon.
Despite not having played in the 15-man code Mason revealed he had been selected to face England at Twickenham and then Wales in Cardiff six days later.
He said: "I am just so excited to be playing rugby and I'm looking forward to the
"I've certainly jumped in the deep end in agreeing to play for the Barbarians without having any rugby experience.
"If you ask all the boys, I've always told them I've wanted to have a crack at rugby (union), and I'm ready to try the best I can to make an instant switch."
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