If you can use Categories in Outlook, you should be able to access them in
code. Look in regedit for this key:
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Categories\MasterList
There is some sample code here that shows how you can retrieve this list:
http://www.cdolive.com/cdo5p2.htm
(See "Read the master categories list of a user" section)
An alternative is to use the Win32API to access the registry, or just Google
for "Visual Basic Registry Class" and you'll find some code that you can
reuse to work with the registry.
There's also some samples here on creating custom command bars and menus:
TIP: Fun with Outlook CommandBars:
http://www.outlookcode.com/d/tips/commandbarfun.htm
Or if you feel like doing some shopping, this add-in might work for you:
CategorieZ - Organize your categories the quick and easy way - About:
http://www.software-solutions.co.nz/aladdins_co/alcoabout.asp
Let me know if you have any questions.