Edinburgh Gunners' Dewey and Borders Reivers man MacDougall have been added to the 35 already enlisted for training at Murrayfield on Monday and Tuesday.
Hadden said: "Ben has made great strides since he joined Edinburgh last season. Rob has earned his call-up with the consistency of his performances."
MacDougall, 28, who has spent much of his career in Australian rugby league, qualifies for Scotland through a paternal grandfather from Glasgow.
He moved from the Gunners to the Reivers over the close season.
He said: "It's my goal to play for Scotland and with the Borders winning three games in a row there's a real feeling that we're starting to get noticed as a team."
Dewey, who celebrates his 22nd birthday later this month, was joint top tryscorer in last season's BT Premiership with 16 touchdowns for Heriot's.
Now in his first full season of professional rugby, he has scored four tries in five Celtic League starts for the Gunners.
Attached to the Scottish Institute of Sport, Dewey also played club rugby at Kirkcaldy and is a graduate of Scotland's age-grade teams and the Scotland Sevens.