Imagining that I am Napoleon is not (normally) imagining an impossibility. It is (or at least may be) just adopting a first person way of imagining Napoleon. The images and bits of narrative using 'I' are intended to refer to Napoleon and his surroundings, in something like the way that a salt shaker can stand for a regiment of troops when the general says "This is the third regiment' while explaining his plans at the breakfast table.