The former Real Madrid youth player has been on goalscoring form for Atletico Malagueno, Malaga's reserve team, who top Group 9 of the Spanish third tier.
Kilmarnock's Greg Taylor and Celtic's Jamie McCart, on loan at Inverness Caledonian Thistle, are the new additions in defence.
St Mirren's Stephen Mallan is among the new crop of midfielders.
Motherwell's Dom Thomas and Celtic's Joe Thomson are rewarded for their loan spells with Queen of the South in the Scottish Championship by being included in the squad for the friendly at Paisley 2021 Stadium on 28 March.
St Mirren's stadium will be a familiar venue for Celtic's Paul McMullan, who had a spell there on loan before his present stint with Dunfermline Athletic, and the Buddies' Lewis Morgan, who are the new forwards along with Harper in Scot Gemmill's squad.
Wolves' Jack Ruddy, who began his career with Real Murcia, is one of two uncapped goalkeepers.
The other is Devlin Mackay, who is on loan to Berwick Rangers in Scotland's League Two from Kilmarnock.
Goalkeepers: Ryan Fulton (Liverpool, loan to Chesterfield), Devlin Mackay (Kilmarnock, loan to Berwick Rangers), Jack Ruddy (Wolverhampton Wanderers)
Defenders: Alex Iacovitti (Nottingham Forest), Zak Jules (Reading, loan to Motherwell), Jamie McCart (Celtic, loan to Inverness CT), Jordan McGhee (Hearts, loan to Middlesbrough), Liam Smith (Heart of Midlothian), Greg Taylor (Kilmarnock)
Midfielders: Chris Cadden (Motherwell), Liam Henderson (Celtic), Stephen Mallan (St Mirren), James Jones (Crewe Alexandra), Dominic Thomas (Motherwell, loan Queen of the South), Ruben Sammut (Chelsea), Joe Thomson (Celtic, loan to Queen of the South)
Forwards: Ryan Hardie (Rangers, loan to Raith Rovers), Jack Harper (Malaga CF), Oliver McBurnie (Swansea City), Paul McMullan (Celtic, loan to Dunfermline Athletic), Lewis Morgan (St Mirren), Craig Wighton (Dundee)

Polk County Sheriff Johnny Moats said the Georgia prisoners, who all have committed minor crimes, would each have their sentences shortened by 25%.
The work crew had been cutting grass at a public cemetery when the corrections officer who was guarding them collapsed in the heat.
The six men used the guard's phone to call police and began performing CPR.
"When that happened, in my opinion, it wasn't about who is in jail and who wasn't," inmate Greg Williams told 11alive.com.
"It was about a man going down and we had to help him," he continued.
The work crew ran to the lone officer after he fell to the ground and removed his bullet-proof vest to perform CPR.
Rather than take his pistol and run off, they used his mobile phone to call for an ambulance.
"None of my guys ran," the officer told WKYC. "None of them did anything they shouldn't have done."
The family of the officer, who does not wish to be identified, bought a lunch of pizza and cupcakes for his incarcerated rescuers the next day.
Sheriff Moats said that "anytime we have a trustee or inmate crew that goes beyond normal duties, we cut them some extra time off".
The inmates' reaction also "really speaks a lot about my officers too, how they treat these inmates", he added. "They treat them like people. Like family."
Their actions come in stark contrast to two other Georgia convicts who were arraigned on Wednesday on charges of murdering two prison guards while being transported between prisons.
Prosecutors say they intend to seek the death penalty for Donnie Rowe and Ricky Dubose, who were captured in Tennessee after a three-day manhunt.
Malaga forward Jack Harper is among 10 uncapped players named in the Scotland Under-21 squad to face Estonia.
[NEXT_CONCEPT]
Six US prisoners are being rewarded for saving a prison officer who had suffered a suspected heart attack.