There is no built-in button for shadow, nor for the outline, emboss,
or engrave settings in the font dialog.
You can put a macro in normal.dot or another global template, and make
a button to run it. This macro will turn on the shadow for the
selected text if it is currently not shadowed, and will turn it off if
it's currently on.
Sub ToggleShadow()
With Selection.Font
.Shadow = Not .Shadow
End With
End Sub
See these articles for help if needed:
http://www.gmayor.com/installing_macro.htm
http://www.word.mvps.org/FAQs/Customization/AsgnCmdOrMacroToToolbar.htm