function f (a::AbstractVector{T}) where {T}
b = Union{Missing, T}[]
push!(b, missing)
return b
end
julia> f([1,2])
1-element Vector{Union{Missing, Int64}}:
missing

if you want to keep the contents of a:

function f(a::AbstractVector{T}) where {T}
b = Union{Missing, T}[]
append!(b, a)
return b
end
julia> f([1,2])
2-element Vector{Union{Missing, Int64}}:
1
2