For Model United Nations, I am planning on being a presiding officer during a conference. However, I have not yet and I am doing ED to the school. Should I put “will volunteer as presidng officer”?
I would not unless you have a firm commitment in place to do so (and then you can put in the conference date). Note that if you do something truly notable after applications are submitted you can email your admissions officer at each school with a brief update.
No, don’t include it. Everyone has big plans for the future. Colleges want to know what you actually accomplished. What if you are hospitalized with the flu the day before? As Happy1 said, it is better to update them later.