I'm reproducing advice from those that have kept them as pets. People have told me that they've fed orphans with goat's milk and bread soaked in cow's milk. I'm not an animal nutritionist, but they both seem like a good start. Marmots are generalist herbivores. Start them on fresh fruits and vegetables as soon as possible.
Marmots love dandelions and cow parsnip.
If they're getting fresh vegetables, they'll probably not need water. Marmots are often infested with mites and fleas. Be careful not to poison them though. If you're aiming to rehabilitate it, I suggest moving it outside into a semi-natural enclosure. Keep feeding it at the box, but let it pretty much explore on its own.