ZBox := Array(1 .. 2, datatype = float[8]):

foo1 := proc (z::(complex[8]), a11::(Array(1 .. 2, datatype = float[8])))
  zi := I*z: z1 := Re(zi): z2 := Im(zi): a11[1] := z1: a11[2] := z2:
end proc:

foo := proc (z::(complex[8]), a11::(Array(1 .. 2, datatype = float[8])))
  cp1(z, a11):
end proc:

cp1 := Compiler:-Compile(foo1):
cp := Compiler:-Compile(foo):

cp(3.3+2.7*I, ZBox):

ZBox;

and the boring warning:

Warning, the function names {cp1} are not recognized in the target language

Have you found more efficient way?