If it's the Freddy 2 it will need something to prevent falls from a height
So if not a full level then a few overlapping shelves. The main thing is that your hammy couldn't fall from the roof right down to the floor if they decide to climb up or monkey bar across the roof. If you need some extra shelves, Ranchhouse cages on ebay do some good ones - I got a pack of shelves from there, or they sell single ones - they just screw to the bars and come with the screw fitments already attached.
Maybe the easiest way is to do something similar to the Alexander cage shelves. ie have a shelf at either side front to back, and then a joining shelf across the back, then a ladder or piece of wood as a joining shelf across the front (resting on the side shelves). Then you will just have a hole in the middle, for access up to the shelves and need something over the hole to prevent falls down.