Like others have mentoined, you need to go to the Qlik Management Console.
There you can duplicate the app of which you are not the owner.
After you duplicated the app, it is visible in your 'Work', an you are the owner.
Now you can edit everything, even add-ons (downloads) are duplicated.
Changing the owner is also possible in QMC, but perhaps not always the best solution.
Multiple owners doesn't seem possible, unless i'm overseeing something.