No, it's not possible. Hypnosis can't alter your body in any way. It can only make suggestions that you change your behavior or your way of thinking.
If you want bigger nipples some people have supposedly had success with something called Supple Nips. It's a small suction cup that more or less stretches your nipples. This is also supposed to make them more sensitive. Nipple piercings can also make nipples more sensitive.
If you want breast growth, you'll need to look at HRT or some other chemical method of enlarging breasts. Some hair growth medication has breast growth as a possible side effect (due to decreasing DHT, a very potent form of Testosterone) and *supposedly* some people have had success with something called Pueraria Mirifica, an herbal phytoestrogen. Be very careful if you decide to try herbs or self-medicate in any way since it's possible you'll fuck up your liver, get blood clots, and generally have a bad time. Best to see a doctor if you want to medicate.