Milk production in women who have not been pregnant is called induced lactation, and it is indeed possible. I don't know where the 80 percent figure comes from, as some women will produce a little bit of milk, some will produce about half of what the baby needs, and a rare few will produce virtually all the baby needs. I don't think an average figure can be universally applied. Interestingly enough, the ability to lactate is unrelated to reproductive history. That means that a woman inducing lactation for an adopted baby after having birthed and breastfed two or three of her own is no more or less likely to produce breast milk than a woman who has never had a child.
Hormones drive the production of breast milk.
Prolactin, a hormone created by the pituitary gland, makes breast milk, and if you stimulate the breast enough to increase prolactin levels, you will get milk. While it would be nice to be able to give someone a shot of prolactin, it isn't quite that easy. Very few drugs increase the amount of prolactin in the body, and most of them are fairly heavy-hitting depressants, such as several of the psychotropic tranquilizers. The nerve endings in the nipple, when stimulated in these ways, signal the pituitary gland to increase prolactin, which in turn stimulates the alveoli in the breasts to begin producing milk. Mothers who wish to nurse an adopted baby should pump several times a day for about ten minutes each time using a hospital-grade double pump.