Cindy Meister, VSTO/Word MVP, on VSTO Microsoft forum, in 2011 re Word 2010:
More exactly, I said there's nothing in the Word API or VSTO that can pick up keystrokes. There's nothing built-in that you can use in a VSTO project to pick up the keystrokes.
You'd either need to use VBA (which isn't part of VSTO) or you'd need to use the Windows API - both of which are not an integral part of VSTO.
In Word, keystrokes can only be mapped to VBA macros, not to other code.
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^].
I would investigate if you can apply a GlobalHook to the Word Application, but, I have no idea is this is possible within the context of an Add-In.