Armitage is due to undergo a medical with the French club on Wednesday after London Irish agreed to release him from his contract.
The 25-year-old, who has five senior England caps but will be making his international Sevens debut at the weekend, will be officially presented by the Top 14 club on Friday, along with fellow new recruit Alexis Palisson, the France wing.
Toulon have agreed to England's request to draft Armitage into their title-chasing squad in place of Greg Barden, who will miss the London and Edinburgh tournaments this month.
England Sevens coach Ben Ryan said: "It's a shame that Greg Barden won't be available for the next two tournaments after he was injured in this season's inter-services games and that's why Steffon is coming into the side.
"He brings a lot of international experience and he has a world-class ability to stay on his feet, win turnovers and dominate the breakdown, which gives us another layer of expertise and makes him the ideal replacement for Greg."
England are second in the Sevens World Series table, 13 points behind New Zealand with two tournaments remaining.
Armitage's last England cap came in the 2010 RBS 6 Nations and he will be putting his senior international ambitions on hold when he moves France.
The Rugby Football Union have already made clear that overseas-based players will not be selected for England following this year's World Cup.
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