several methods can be used.

below procedure looks for col A cell values in col B and if not found writes to the first blank cell in col B.

[VBA]
Sub AddMissingValues()

For i = 1 To Range("A" & Rows.Count).End(xlUp).Row
If Application.CountIf(Range("B1:B" & Range("B" & Rows.Count).End(xlUp).Row), Range("A" & i).Value) = 0 Then
Range("B" & Rows.Count).End(xlUp).Offset(1).Value = Range("A" & i).Value
End If
Next

End Sub
[/VBA]