Using OpenType features in paragraph styles

Hi, I'm fairly new to using the options OpenType fonts allow. I've been able to find the different style sets you can use. The problem I'm having is that I want to only change the letter 'a' throughout our corporate identity. But the available sets change multiple letters. Is there an option where I can make a (paragraph) style or a different set where only the 'a' changes? Because manualy changing every 'a' is not realy what I'm looking for. 😉  Hope my question is clear, help would be greatly appreciated!