This is not possible, mainly for safety reasons having to do with the mechanics of the microwave. If the food container does not fit, then you might need to apportion the food and put it into smaller containers to heat it.
== As truth is a most valuable commodity, it is important to vend it. There are ways to get a microwave to run with the door open. To say that it is not possible is incorrect. But, at the same time, it is foolish and dangerous to even consider this. There are several questions (and answers) posted on the boards dealing with radioactive materials where individuals asked about "getting a hold of some" or the like. This question falls in the same catagory. There is a point here, so let's make it. It is a broadly accepted fact that when an individual wants information, even if it be of the same nature as is being asked for in this question, that person will scour the countryside to get it. It's just that simple. But by giving out the information here, it is believed that (at least in this case) the person, who has a high probability of finding out what he wants to know anyway, will also come away fully warned and pointedly aprised of possible consequences associated with moving forward to investigate something which has been asked about. Running a microwave with the door open causes the microwave radiation to exit the cavity and radiate out into the surrounding area. It will be reflected and scattered in a "normal" way. Anything biochemical (living) around it will absorb some of this energy. And this presents a danger to the living tissue. It will warm, but can be killed by the heat. Heat causes chemical changes in biochemical materials. That's what "cooking" does, too. Proteins dissociate (break apart) and there a zillion more reactions possible. Operating a microwave with the door open is an invitation to a disaster. It's like checking the fuel level in a gas can with a lighted match. Please don't even think of trying this. It is foolish beyond words.