"I'm under no illusions how tough it's going to be to make the final selection but I'm looking forward to getting started and showing how much this opportunity means to me and my family," Hardie added.
Scotland head coach Vern Cotter must reduce his squad to 31 following four warm-up games in August and September.
"We're pleased to welcome John into the group and look forward to him intensifying the competition for places in an already fiercely contested area of our squad, Cotter said.
"Depth is crucial to any World Cup campaign and it will be vital to see the contributions of the entire squad in our four warm-up matches, before making a decision on the final 31."
Scotland play four warm-up matches against Ireland, Italy (twice) and France before their opening World Cup game against the United States in on September 20.