My parents neighbour did this with a double colour bond garage, they just removed the roller doors and replaced with sliders, frames and gyprocked the inside and installed an ensuite and kitchen. It gets really got though
The first thing you would have to do is find out the mpa they used in the concrete. Being a shed the slab would not be substantial enough to fold the strength of a 2 storey house.
If it has suitable mpa and providing the slab is the correct depth the plumber could tench the concrete to run...