Hi, Kelly. I had flat/inverted nipples, too, and they popped out naturally during my pregnancy. I'm not sure at what stage that happenned, but I remember that by the time my son was born, my nipples were ready to feed him. Actually, the first stages of breastfeeding will also help to get the nipples ready.
Even a nipple that isn't inverted or flat goes through changes during pregnancy and breastfeeding. They all get larger, as you can imagine.
I don't know what a breast shell is, so I can't help you there, but if you want to condition and encourage your nipples, you can try smoothing on and rubbing in something like olive oil or some other non-toxic or non-harmful oil. You can rub it into the nipple and areola (the dark area around the nipple) and gently stroke the whole area outward. This seemed to help me somewhat, but nature usually takes over and gets the nipple ready with or without much intervention.
I do recommend some kind of conditioning before giving birth, though, because the nipples can really hurt for the first couple of weeks of breastfeeding. I almost quit because of the pain, but one day, it just stopped hurting. Olive oil can't hurt you, and in fact, the most famous nipple cream is made from olive oil, so that's what I would recommend for conditioning the nipples before birth.
Best wishes for a healthy pregnancy, a healthy birthing, and a wonderful life!
Peace,
Syl