Customizing keyboard layout slightly

Hi! I've been using SwiftKey for quite a while now, but I'd like to see a few keyboard layout customization options. I understand that moving the letters around would mess with Flow, but that's not what I'm asking. I'd like to add some additional letters or symbols to the long-press pop-ups on the letters, which shouldn't mess with the layout too much. For example, I'd like to add the "æ" symbol to the long-press pop-up for the letter A. Is it possible to do so?