Possible and happens even a lot later than that at around a year to 18 months I have heard of women able to breastfeed. There are a fair few resources although a lot centre on bringing in the milk and assume the baby is younger. However on a site called kelly mom they have a couple of stories from parents who have breastfeed older kids.
The hardest part would not be the childs but yours. If the child is adopted your breasts have not prepared themselves for breastfeeding and do not have the proper set up. Wet nurses existed because they had recently given birth and were still producing, as long as something is drawing the milk the milk will be produced, but if there's never been a physical need to milk it wont. I'm not sure if there's a way to induce production but if there is I think it would be a fantastic idea for the bonding and nutrition.
Re: Breastfeeding adopted child