Book Designer allows you to insert extended ANSI characters. It is not necessary to cut and paste from Word to fix accents.
With the book language set to German, use Insert... Special Symbols.
This isn't useful unless you know what code you need, so here's a link that shows the extended characters:
http://www.fingertipsoft.com/3dkbd/ansitable.html
All characters appear as documented when publishing an .lrf, except for 164 and 173.
You can also insert a symbol, and use cut and paste to use it in the find/replace dialog. I use this to replace the vertical keyboard-style single and double quotes with the curly versions (145 - 148).