Fostering is a good idea, If you already happen to have a doe bred and ready to deliver around the same time, BUT if you just received her that would violate quarantine and put your other buns at some degree of risk for whatever is affecting her. It's a tough call.
There are SO MANY things that can cause a rabbit to feel thin, there is no way to guess the cause with just that...
If she were mine, I'd be doing my best to address the cause of the weight issue instead of worrying so much about the litter.
I would probably give her the chance to raise them, UNLESS I felt the stress of raising them might harm her or her immune system, and compromise the doe.
If it's doe VS kits, I always choose to save the doe and do not hesitate to euthanize kits. That was a hard lesson for me to learn. :|
Keep an eye on her digestion. If she is doing well on the feeds you are giving, it shouldn't be too hard too give her enough calories to feed a few kits and recover her weight a bit.
If you are not already feeding lots of grass hay, strongly consider doing so. You can give a rabbit infinite calories, but it won't help at all if she can't digest what she is eating.
Calf manna is very high in carbs, while boss and pumpkin seeds are higher in oil content. A nursing doe can turn dietary oil directly into milkfat, and I feel that feeding a sparing amount of those oily foods is very good if you do not want a lactating doe to tap into her own fat deposits.
Oats are a good carb source that is also high in soluble fiber. If you see any digestive upset, I'd switch off calf manna and give oats instead.
You might want to treat for cocci or intestinal parasites before the litter arrives, if you observe any symptoms pointing in either direction.
Also consider weaning the kits closer to 4 or 5 weeks instead of later if she is still on the thin side, since supplementing milk to older kits seems to a very big caloric draw on a doe.
