Custom Solution for MSWORD
To customize shortcut keys in MSWORD, choose INSERT, then SYMBOL. You will see the following field.

From here, select the special character you want, then press the SHORTCUT KEY button.

Click in the Press new shortcut key field, then simply enter in your new key combination. I suggest making it simple, like CTRL-U for ü. After you have entered in your combination, click ASSIGN to set it.

Close this window, go back to the INSERT SYMBOL menu and cycle through the following special characters in German.(ü ö ä, Capital umlauts Ü Ä Ö, ß and €)