He joins the squad as an injury replacement for Gavin Lowe who dislocated his shoulder in Dubai.
The Glasgow Hawks man - who is attached to the Warriors - could make his senior debut in Cape Town having trained with the Scotland squad in the pre-series preparation camp.
Scotland Head Coach Calum MacRae said: "It's tough losing Gav, he's developed into a top performer and demonstrated in Dubai that he has the potential to influence a game.
"He is getting scanned this week and I remain hopeful that the injury isn't too extensive.
"Robert performed well during the preparation camp, he has a lot of potential and will get up to speed during the training week.
"It will be a special moment for him winning his first senior cap and I'm sure he will take confidence from the way George Horne has slotted in and performed."
Scotland will open the competition in South Aftrica having been drawn in Pool A alongside Samoa, Uganda and Wales.
Scotland 7s fixtures in the second leg of the World Rugby HSBC Sevens Series in Cape Town, Saturday 10 December & Sunday 11 December (ko times in GMT)
Saturday 10
8.15am - Scotland 7s v Samoa 7s
11.36am - Scotland 7s v Uganda 7s
3.19pm - Wales 7s v Scotland 7s
Sunday 11
TBC
Scotland 7s squad to compete in Cape Town
1 Scott Riddell
2 Dougie Fife
3 Fraser Lyle
4 James Johnstone
5 Nick McLennan
6 Mark Robertson
7 Jamie Farndale
8 Joseva Nayacavou
9 George Horne
10 Scott Wight
11 James Fleming
12 Robert Beattie