Actually, you CAN add a custom control using VBA by editing the officeUI customization file as Outlook is started up.
Here is a sample solution...
experts-exchange solution: HOW-TO-Manipulating-Office-Ribbon-Bar-only-with-VBA
It is not perfect, as some error checking or checking to see if the custom tab, group and / or button already exists before trying to add it in again. Also, it seems that the code presented here doesn't quite handle the situation where there have been no customizations to the ribbon already made... at least in Outlook 2010.
Also, using similar methods of editing the same file, you can add custom buttons to the QAT (Quick Access Toolbar).