Glasgow Warriors duo John Beattie and Thom Evans, Edinburgh Gunners' trio Rob Dewey, Alasdair Dickinson and apprentice Ross Rennie have all been selected for the two-day session next week.
Border Reivers' full-back Calum MacRae and Northampton Saints full-back Robbie Kydd have also been included, while Phil Godman and Allan Jacobsen, who both play for Edinburgh, and Warriors pair Graeme Morrison and Euan Murray have been promoted from the A squad.
But there is no place in the training squad for stand-off Gordon Ross and scrum-half Sam Pinder ahead of the Tests against Romania, Pacific Islanders and Australia in November.
Hadden said: "It was the toughest ever squad to pick at this stage of the season compared to similar squads of this size I have picked in the past, simply because we have more good players outside this squad than before.
"Though the pro-teams are not performing as well as I would have liked overall, there are many more Scots players now playing at a level that intensifies competition.
"I can honestly say that most of the players are in good fettle despite of some of the results with their teams."
But Hadden has also called on flanker Rennie, 20, and winger Evans, who is a year older, to make an impact in time for the World Cup next year.
He added: "I want to see them operating in this company.
"I see this is an opportunity for them to put a marker down for the World Cup."
Simon Danielli, Rory Lamont, Ben MacDougall, Jon Petrie and Simon Taylor have not been considered for the training camp due to injury.