One way:
Option Explicit
Sub auto_open()
On Error Resume Next
Application.CommandBars("cell").Controls("Format Painter").Delete
On Error GoTo 0
With CommandBars("Cell")
With .Controls.Add(msoControlButton, temporary:=True)
.Caption = "Format Painter"
.FaceId = Application.CommandBars("standard") _
.FindControl(ID:=108).FaceId
.OnAction = ThisWorkbook.Name & "!formatpainter"
End With
End With
End Sub
Sub FormatPainter()
Application.CommandBars("standard").FindControl(ID:=108).Execute
End Sub
If you put this in a workbook and store that workbook in your XLStart folder,
you'll have added this each time you start excel.
You could also just open that workbook when you want to use it, too.
If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm