Outlook 2011: Keyboard shortcut to assign categories?
Yes, you can do this.  


Open System Preferences.
Select Keyboard.
Select Keyboard Shortcuts.  
Select Application Shortcuts in the left navigation pane.
Click the + button to create a new shortcut.
Choose Microsoft Outlook.app in the Application drop-down menu.
Enter the exact name of the menu command you want to add. E.g. for an Outlook category named "Project X" enter that category name, "Project X." 
Click in the Keyboard Shortcut field and press a unique keyboard sequence such as Control-Option-T. 
Click the Add button.


For some more ideas about how you could add keyboard shortcuts to Outlook:Mac, check out "Unleash new uses for keyboard commands in Outlook for Mac" in the Office:Mac Help Blog.