There is a way to add a custom ribbon into Outlook 2013 with the help of ADODB.Stream
I'm working with this solution at work since some years - but I was not able to apply it also at home.
First of all I'm preparing a text file that has the XML-strukture in it:
Dim Stream As Object
Dim FSO As FileSystemObject
Dim tsZwischenspeicher As TextStream
Set Stream = CreateObject("ADODB.Stream")
Set FSO = CreateObject("Scripting.FileSystemObject")
strPfad = "C:\Users\" & (Environ("username")) & "\AppData\Local\Microsoft\Office\"
strSpeicherpfad = strPfad & "olkexplorer.officeUI"
strTempSpeicherpfad = strPfad & "olkexplorer.temp"
...
tsZwischenspeicher.WriteLine Anführungszeichen("<mso:customUI xmlns:x1='http://schemas.microsoft.com/office/2009/07/customui/macro' xmlns:mso='http://schemas.microsoft.com/office/2009/07/customui'>")
tsZwischenspeicher.WriteLine Anführungszeichen("<mso:ribbon>") & vbCrLf
tsZwischenspeicher.WriteLine Anführungszeichen("<mso:qat>")
tsZwischenspeicher.WriteLine Anführungszeichen("<mso:sharedControls>")
tsZwischenspeicher.WriteLine Anführungszeichen("<mso:control idQ='mso:FilePrint' visible='false'/>")
Then the generated XML-File could be transfered into Outlook via ADODB.Stream:
'Eine neue Fußzeile erstellen
tsZwischenspeicher.WriteLine Anführungszeichen("</mso:tabs>")
tsZwischenspeicher.WriteLine Anführungszeichen("</mso:ribbon>")
tsZwischenspeicher.WriteLine Anführungszeichen("</mso:customUI>")
'Zwischengespeicherte Datei schließen
tsZwischenspeicher.Close
Stream.Open
Stream.Type = 2 'text
Stream.Charset = "utf-8"
Stream.LoadFromFile strTempSpeicherpfad
FSO.OpenTextFile(strSpeicherpfad, 2, True, True).Write Stream.ReadText
Stream.Close
Outlook has to be restarted and the new Ribbon will be displayed.