If I apply for the SIT global dormitory and get accepted, is it required that I move in to the dormitory or is it okay if I refuse it?

It is better not to refuse once you get accepted as getting a room in the dormitory is highly competitive.  
However, the best is to ask the dormitory directory at  
sit-dormitory@ow.shibaura-it.ac.jp
If you couldn't find an answer to your question or concern,
-For enrolled students: you can inquire us through the Scomb we shared when sending you the Before Arrival Guideline.
-For prospective students: you can inquire from
here.