As far as I know this is not possible. Probably the easiest solution would be to duplicate the attribute and replace all 'à’ with normal ‘a’ then use this duplicate attribute to search.
-Andrej
Another approach would be to use Xpath like this:
[contains(text,'à') or
contains(text,'ä')]
where ‘text’ is the name of an attribute on my entity. Then you can add all the characters you need to search for.