The national coach is directing operations for the second string during the tournament, which will take place in England over the next two weeks.
The Scots reached the final of when it was held in North America last year, notching a famous win over their English counterparts on the way.
They fell at the final hurdle to a classy New Zealand Maori side but are hoping to go one better this year when their campaign gets under way against the USA in Henley next Wednesday.
But for the players involved there is a far bigger prize at stake.
Speaking after Thursday's squad session at Murrayfield, Hadden said: "I told the players before we started training that there are 26 players who are definitely coming to train with the full Scotland squad on June 4th, but we're going to extend that to approximately 38, so there are about 12 places for the extended World Cup squad up for grabs.
"So it is a big opportunity for this group and that is why it is me, Alan Tait, George Graham and Mark Bitcon here, and not a separate Scotland A coaching squad taking this team.
"We need to see these players at first hand and we need to have them working with our structures first hand as well.
"The reason they are here is because they have impressed me, but I want to be impressed first hand. I need to make sure that they are up to the mark.
"I have promised the whole squad that they will get a fair crack of the whip over the next three games, because that is why they are here - they are here to try to impress me.
"To be absolutely honest, there are some guys in the squad who are closer to international selection than others, but there is still an opportunity for everybody to either make me change my mind, or convince me that I was thinking along the right lines in the first place.
"I learned a lot just watching that today (Thursday). There was a significant number of things that I picked up just by watching them go through their paces there, so there will be one or two adjustments for tomorrow (Friday), and again on Monday, before we are ready to play the USA on Wednesday."