Barclay missed the opening Six Nations games after being out for two months with a hamstring injury, but has since made his competitive comeback with Glasgow.
The additional players join all but one of the 23 who were stripped for the 34-10 triumph over Italy on February 9 with prop Euan Murray unavailable on Sunday because of his religious beliefs.
Hooker Dougie Hall is back training after being forced to miss the Italy game with a knee injury.
Fellow Glasgow forwards Jon Welsh and Tom Ryder have also been called up along with Sale pair Alasdair Dickinson and Richie Vernon and Edinburgh lock Grant Gilchrist.
Glasgow backs Sean Kennedy, Alex Dunbar, Peter Horne and Peter Murchie are also included in the extended squad along with Worcester winger Nikki Walker and Bath fly-half Tom Heathcote.
Interim head coach Johnson is due to name his team on Wednesday.
- Sports & Recreation