Thorman?s call is reward for his excellent form of recent weeks, which has helped the Giants reach the Powergen Challenge Cup final.
The 25-year-old also has extra cause for celebration after agreeing a contract extension with the Giants to remain at the club until 2009.
Thorman said: "They say good things happen in threes, so hopefully the next one is the Challenge Cup final.
"It is good to have your efforts recognised and it is particularly good to have the contract issue sorted out."
England international Thorman is uncapped by Britain despite being named in squads twice previously.
Giants coach Jon Sharp, also the Lions assistant boss, said: "His international recognition is well deserved and is a big boost for both Chris and the club."
The Tynesider becomes the 43rd member of coach Brian Noble?s squad, a party which will be trimmed to 25 - with 10 on standby - on September 25.
It is intended all 35 will travel Down Under to prepare for the tournament in October with the 10 standby players, if not needed, returning to play for England in the European Nations Cup.
The squad will fly out in two groups depending on which players are involved in the engage Super League Grand Final on October 14.
Those not playing will travel early and prepare for a warm-up match against a Newcastle select side in Newcastle on October 20.
Britain begin their Tri-Nations campaign against New Zealand in Christchurch on October 28.