My concern would be that if the "supplement" was very far off from what they're used to consuming, it may cause some digestion difficulties since the gut flora are adapted to mice. And I have no idea where to buy tubs of rodent fat.
I'm not sure that rodent fat is necessary, I would think that any animal protein and fat would be sufficient. With my processor, I could liquify a single piece bacon and boost an adult F/T mouse to the caloric equivalent of a small rat. What do you think?