Wait. I’ve tried that on the Julia interpreter and found that its type is 1×3 Matrix{Int64}.
That means that the most straightforward solution to my problem would be
a = Array{Int}(undef, 1, 3)
b = Array{Int}(undef, 1, 3)
# . . . read a and b from files . . .
m2x3 = vcat(a,b)
Thank you for the idea.