I think it is do-able, are you handy with a saw and non toxic glue? You could make a second level by creating a 'shelf' out of wood (very simply one sheet of wood) held up with supporting legs? Or perhaps you could create a mezzanine floor using plastic...if you take a look at the photographs in our Member Commercial Cage Gallery, the very first photographs by MrsM show a mezzanine floor in her cage, if you keep scrolling down you'll see some other examples too:
I'm not a hundred percent sure on that part, as i have never made one before however i would assume you'd use Corruaged Plastic which you would bend or use a craft knife (?) to bend the plastic into shape, and then use cable ties to hold it in place...
I have a shop bought cage, its quite small but it is inside a larger playpen, they have access to all the space, all the time. It works well for my girls.
You can see pics on my thread 'My girlies christmas cage'.