For pregnant or nursing I like to give:
- dried milk/milk powder/milk replacer (you can use any kind, but the cheapest will be an "all milk replacer" from a tractor supply or farm and feed; the only downside is that they sell them in sized bags for entire litters or for orphaned farm animals, so you will have A LOT which is the only benefit to getting a smaller more expensive type)
- canned milk (but they will probably get bedding in it or knock it over)
- yogurt (again, same spill problem as milk)
- quality dog food kibble (out of all of their food, it is always their FAVORITE to eat; I always put that in their food mix though)
- boiled or scrambled eggs
- cooked meat

aside from Milk Powder, all of these should be taken out of their cage if not eaten.