try this,
modify the position value as per your need..
var array1 = ["I", "II", "III"];
var array2 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18];
var arrayout = [];
var position = [1, 10, 17];
var i1 = 0, j1 = 0;
for (var i = 0; i < array1.length + array2.length ; i++) {
if (position.indexOf(i + 1) != -1) {
arrayout.push(array1[i1])
i1++;
}
else {
arrayout.push(array2[j1]);
j1++;
}
}