Scotland head coach Gregor Townsend kept two places aside when he named a 42-man squad three weeks ago and lock Cummings and centre Steyn have forced their way in after a strong finish to the season, which culminated in Warriors' narrow PRO14 final defeat to Leinster on Saturday.
Townsend said: "Scott has played very well in some crucial games for Glasgow and has really taken his game to the opposition. He was impressive in his ball carrying, defence and his all-round work-rate on Saturday against Leinster and we look forward to working with him and the other four second-rows in our squad."
Steyn has shone for Glasgow at centre, but Townsend said he is being primarily considered as a winger.
"Kyle has made an excellent impact since his debut for Glasgow earlier this season and has displayed strong defensive capabilities as well as a providing go-forward when carrying ball," Townsend said.
"We have others in our squad that we view more as centre/wings but I'm sure over the course of the summer Kyle will get the opportunity to show what he can do in both positions."