Tanks for your tips guys. I can't make my Universal print 'ö' instead of any other key. At this point I must give up for now.
Now i think it's not possible to display any of our special characters via redirecting/rewriting key codes. It might be because the keyboard driver in WM5 is too stupid to do this. It doesn't support the key codes of special characters, those codes (like 0xf6)are simply 'empty'. (I think it's the 'keybddr.dll')
Now I still have the option to get my wanted characters via sym button with my edited sym.txt file. Unfortunately it takes much more taps, but it at least works.
I think the only way to reassign keyboard keys to any of our special letters (aéíüűöőú) with this dll leads not via keycode rewriting, but some other method.
Thanks for your help, Tom!