Henry Pyrgos has also been called in as the squad prepare for the opening match against Italy.
Another three Glasgow players - flanker Matt Smith, centre Alex Dunbar and lock Rob Harley - have been released.
Smith injured his shoulder in Warriors' 9-3 win over Ospreys on Friday, while wing Lee Jones sustained a knee injury.
Zander Fagerson and Fraser Brown will also join the squad in camp this week.
Tight-head Fagerson and hooker Brown are close to comebacks from ankle and knee injuries respectively.
McDowall, 20, has represented Scotland at Under-18 and Under-20 level but has played only eight senior games - including six starts - for Glasgow.
The 6ft 4in centre, who can also play in the back three, signed his first professional contract with Warriors in December.
Hughes, who won the last of his four caps against Australia in June 2017, has recently returned to action after spending much of the last 18 months on the sidelines.