The pair have been drafted in after James Forrest of Celtic and Birmingham's Chris Burke pulled out through injury.
Forrest was forced off the park towards the end of Celtic's 4-1 victory over Ross County last Saturday while Burke picked up an ankle injury against Huddersfield on the same day.
Conway, a regular in Brighton's side this season following his loan move from Cardiff, has been capped five times, the last when coming on as a substitute towards the end of Scotland's 3-2 defeat by England at Wembley in August.
Mackay-Steven, who scored one goal in three Scotland U21 appearances, has yet to make his full international debut despite being included in several squads since breaking into the Dundee United first team.
Robert Snodgrass and Steven Fletcher did not train with the rest of the squad on Tuesday morning but are expected to take part on Wednesday.
Coach Mark McGhee said: "Snodgrass had taken a blow on his nose and Fletcher has a programme that he's working to in his recovery period coming out of the injury he had.
"We are respectful of that. Both will train tomorrow."
Scotland take on the States at Hampden on Friday before heading to Oslo to face the Norwegians at the Aker Stadion in Molde on November 19.
You can watch Scotland take on the United States live on Sky Sports 2 from 7.30pm this Friday