The 24-year-old will make the cross-code switch at the end of the Super League season after being earmarked as replacement for centre Seilala Mapusua, who joined Kubota Spears in 2011.
Sa, who has played in the centres and the back row in the 13-a-side code, has agreed a three-year deal with the Exiles.
He has made 39 appearances for the Dragons, having moved to France in 2009 after a previous spell at the Sydney Roosters, while he has been capped by both New Zealand and Samoa.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
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