What do you mean by offset or sustainable? What are the impacts of junk food are you trying to avoid? Effects on health? Weight gain?
It is possible to have a small portion of your diet consist of junk food and still meet your health goals. As long as they do not put you in a surplus or displace needed macro or micronutrients you should be okay.
Junk food is sustainable without all those things, depending on how much. What exactly are your “negative” outcomes of junk food you’re trying to avoid, that’ll make it easier to answer if those things can offset it