You need a special formula that is meant for marsupials as they cannot tolerate the lactose in milk-based formulas. How old is the baby? Was it thrown for the mother's pouch or pulled to handrear? What species and what sex is it? At what temperature and humidity is it being kept? Will you be the only caretaker? Marsupials are routinely handreared by humans but it takes a lot of work. They also need a special teat that is specific to marsupials and their mouth structure. I've raised a few over the years and it is labor-intensive.