You will have to create a new Action for each one. (this only changes the field that contains the text to what is needed, this can be changed to set another field, but will require tinkering)
In the Condition, set it to "NameOfField" contains "LetterYourLookingFor"
In the Actions, set it to concat(substring-before(., ""LetterYourLookingFor"), "LettersYouWantItToBe", substring-after(., "LetterYourLookingFor"))
Ex:
Condition - FieldA contains "ä"
Action - Set FieldA = concat(substring-before(., "ä"), "ae", substring-after(., "ä"))
Setup each character(s) with their own Actions so that each character(s) is tested.
I attached a sample.