As far as I know there are a number of us that overwinter mini nucs and there was a thread to this effect last year but the search engines do not find it, you need to fill the bottom box with frames or starter strips, make an eke and feed from there, varroa treatment is carried out various ways but i cut one of HM's oasis slabs into nuggets last year, and obviously they need keeping an eye on through the winter and topping up the fondant as required. Many on here will say its a waste of time, for me it gives me spare queens at a time when no others are available next spring.