The 32 year old has played 18 tests, including all four pool matches at the 2011 World Cup, but played for Samoa since 2013.
Head coach Stephen Betham says when they selected the team to play the All Blacks and Pacific Nations Cup on form, Williams had just come back from injury.
The Stade Francais custodian will start for the Barbarians against his countrymen this weekend and says he's still extremely passionate about the game back in the islands.
