You can serve sardines packed in sunflower or olive oil solo, as a treat or pour it oil and all over her standard feed to tempt her to eat a balanced diet with the protein & caloric boost. Sunflower seeds will also help her to pack on some weight (mine prefer them without the shell). Shredded cheese, or preparing her scrambled eggs with some cheese sprinkled on top afterward will also provide some added calories and if you don't COOK the cheese, she can get some probiotics out of the deal too; The good bacteria survive the aging process in some cheeses, including Gouda, mozzarella, cheddar and cottage cheese. Just some ideas on how to fatten her up.