Application Level

Excel Add-in with a Custom Ribbon

Using an application specific add-in (.xlam) and embedding the XML Markup into the Open Office XML file.

Word Add-in with a Custom Ribbon

Using templates (.dotm) in Word - create an external customisation file and point the open xml format at it.

Outlook and Access

Implementing an application level add-in in these applications can be slightly different.

Add-in Customisations

When the application starts the add-in loads and runs the XML markup code.
The Ribbon customisations are always applied and available (until the add-in is removed).
this is typically from a procedure that returns the xml markup from an external file or from xml generated in the code.