' Create a new workbook
' open the vb editor, add a module, add this code
' Attach a new toolbar button to Sub AddComma()
' Return to your target workbook
' select cell in the column you want commas
' click the button

Sub AddComma()
 intCol = ActiveCell.Column
 intLRow = GetLastRow(ActiveCell)

    For i = 1 To intLRow
        Set oCell = Cells(i, intCol)
        curVal = oCell
        
        If curVal Like ",*" Then
            msg = "The value in Cell " & oCell.Address & " already has a comma"
            
            Response = MsgBox(msg, vbOKCancel)
            If Response = vbCancel Then
                Exit Sub
            End If
        End If
        
        oCell = "," & curVal
    Next
End Sub


Function GetLastRow(rngInput As Range)
    Dim WorkRange As Range
    Dim i As Integer, CellCount As Integer
    Application.Volatile
    Set WorkRange = rngInput.Columns(1).EntireColumn
    Set WorkRange = Intersect(WorkRange.Parent.UsedRange, WorkRange)
    CellCount = WorkRange.Count
    For i = CellCount To 1 Step -1
        If Not IsEmpty(WorkRange(i)) Then
            GetLastRow = WorkRange(i).Row
            Exit Function
        End If
    Next i
End Function