St Johnstone goalkeeper Zander Clark will likewise join the crew to supplant the harmed David Marshall.
Stewart has scored 22 objectives in 38 games for the English League One side this season.
Halkett will cover focal safeguard after Scott McKenna pulled out, with Liam Cooper and John Souttar sidelined.
Both first-time call ups could make their introduction in the cordial with Poland on Thursday, with Scotland then playing either Austria or Wales the next Tuesday.
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In the mean time, commander Andy Robertson is an uncertainty subsequent to missing Liverpool's FA Cup quarter-last win against Nottingham Forest with ailment.
Stewart played for Ayrshire junior clubs Ardeer Thistle and Kilwinning Rangers following quite a long while in the star youth framework with St Mirren, Celtic and Partick Thistle.
He then acquired a move back to the senior game with Albion Rovers and had stretches with St Mirren and Alloa Athletic prior to coming to noticeable quality with Ross County, where he scored multiple times in 82 appearances to procure a move south.
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