Type this in cell B1, and copy down...
="X"&A1
This would also work:
=CONCATENATE("X",A1)
And here's one of many ways to do this in VBA (Disclaimer: I don't code in VBA very often!):
Sub AddX()
Dim i As Long
With ActiveSheet
For i = 1 To .Range("A65536").End(xlUp).Row Step 1
.Cells(i, 2).Value = "X" & Trim(Str(.Cells(i, 1).Value))
Next i
End With
End Sub