I don’t think you can yet. There will be a premium version coming sometime next year (I can’t wait) that should support something along those lines.
You can add “Tokens” to the hero and then have the game check to see if your hero has it to do various things: for example have the hero get “Armor 5”, adds 5 health, then later have the hero get “Armor 10”, if the hero has the token “Armor 5” you would have the game add 5 health, remove token “Armor 5” then add the token “Armor 10”, but if the hero lacks “Armor 5” then have the game add 10 health for getting “Armor 10”. There would be no graphic or sprite changes from these (yet), but it’s as close as we can get at the moment.
Cheers!