関数名とかは適当

fill.vb

Sub test()
    Call fillXtoY("", 0)
End Sub

Sub fillXtoY(x, y)
    For i = Selection(1).Row To Selection(Selection.Count).Row
       For j = Selection(1).Column To Selection(Selection.Count).Column
            If Cells(i, j) = x Then
                Cells(i, j) = y
            End If
        Next
    Next
End Sub