There's no real thing as a global context menu. This is purely because different applications can use different edit controls. The one we see in Notepad would probably be the closest to what we could call a generic menu, which you could modify (if you had the source) using something like this.
What you could use is Notepad++ instead, which is far more customisable that the stock Notepad, and has support for all kinds of plugins support.
As for office, if you're on 2003 you can simply
- right clik the menu area
- Click Customise
- Choose the Toolbars tab
- Check Shortcut Menus
- Find the menu you wish to edit in the Text menu
If you wanted a programming method instead try this.
As for office, if you're using 2007/2010 (the ones with the Ribbon UI), then you could do follow this tutorial or this from Microsoft themselves.
[Picture sourced from link above]
Having said all that
I don't think right-click menus are the way to go. The best way I'd go about something like this is using Autohotkey. Of course it really does depend on what you're after but there's heaps of resources out there that could help. Autohotkey doesn't add context menus but you can use shortcuts to perform tasks, such as this (by David James Miller)
Clip0 = %ClipBoardAll%
ClipBoard = %ClipBoard% ; Convert to plain text
ClipBoard = %Clip0%
VarSetCapacity(Clip0, 0) ; Free memory
which basically pastes any text as plain text (removes formatting like fonts, italics etc).
Hope you find something useful here!