Basically, you feed a normal good diet with plenty of meadow or timothy hay with some alfalfa hay added (about a handful), a normal or only slightly larger amount of mixed fresh veg that includes a daily double potion of a high vitamin C veg or herb (like two slices of pepper instead of one or more broccoli, or a few sprigs of fresh coriander), and not more than a maximum of 40g of pellets per day, rather a bit less. Too much pellets can cause the pups to grow too large and cause problems at birth; pregnancy toxaemia (blood poisoning) is also more frequent with sows on a high pellet diet. Do not overfeed your mummy to be!
Please be aware that the additional amounts of calcium and vitamin C needed are actually not all that big. Sows on a normal good diet have no problems with giving birth to perfectly healthy babies!