Henry Pyrgos has also been called in as the squad prepare for the opening match against Italy.
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.