Legal side of add-ons
What I mean, is that the add-on is basically the auto-nav with a different template applied to it. Do I have to rewrite the auto-nav code or can I use that as a base for the add-on?
The add-on would be developed to be sold in the Marketplace so needs to abide by the rules.
In short, yes you can use any parts of the code, but you have to keep the copyright notice of the holder.
More about the MIT License on wikipedia: