Within the Sims 4 it might not be an option to breastfeed an adopted baby; that in no way means that isn't an option in real life. There are adoptive mothers who breastfeed their adoptive children. Also there are ways to do this with the help of your doctor. I have done my research on the subject for a story I was writing. You don't have to search very hard on the internet to backup what I'm saying.